Keep Documents and App Data in Sync
With iCloud Drive, Macs have Finder access to online file storage in iCloud. iCloud Drive gives you a single location where you can (in theory) see any of the documents that any of your apps have stored in iCloud or that you’ve manually copied to iCloud Drive. You can organize files in any manner you choose, even creating your own folders. Any Mac app can store documents there, even if it hasn’t been designed specifically to work with iCloud. And you can access iCloud Drive not only from your Mac but also through the iCloud website, in the Files app for iOS, and from many iOS apps.
iCloud Drive’s predecessor, Documents in the Cloud, still exists—but only for users running 10.9 Mavericks or earlier, or iOS 7. Because it’s essentially obsolete, however, and because most users have upgraded to 10.10 Yosemite or later, or iOS 8 or later, this book no longer covers Documents in the Cloud.
To learn all about iCloud Drive, read the next topic, Use iCloud Drive. In addition, you’ll want to read Use In-App Data Syncing, later in this chapter, for information on working with apps that don’t rely on documents but do need to sync data across devices.
Use iCloud Drive
iCloud Drive is cloud-based storage you can access on any of your devices—either independently (for example, in the Finder on a Mac, or in the Files app on an iOS device) or from within apps that have been updated to support it.
With iCloud Drive, master copies of your documents are stored in the cloud. Your Mac normally maintains a local copy of these documents, too—you can’t turn off syncing—and any changes you make to the contents of your iCloud Drive on one device immediately sync, via Apple’s servers, to the others. (However, your Mac might delete the local copies of some files if you’ve enabled Optimize Mac Storage and your disk starts running low on free space.)
That sounds simple enough, but iCloud Drive has quite a bit of hidden complexity and doesn’t always make sense. You’ll be able to use iCloud Drive more effectively if you understand at least some of Apple’s logic. After I tell you how to turn on iCloud Drive (next), I’ll give you a quick overview of how it ought to work in normal situations (see Explore iCloud Drive). Then I’ll delve into what actually happens (see Understand iCloud Drive Peculiarities), which can be quite a bit different.
Activate iCloud Drive
For most users, iCloud Drive was already activated at some point within the past few years—typically while upgrading to a new version of macOS or iOS. If you deliberately skipped enabling iCloud Drive for some reason, you can activate it whenever you’re ready by going to System Preferences > iCloud (Mac) or Settings > Your Name > iCloud (iOS) and turning on iCloud Drive.
There are a few other iCloud settings you can change. While the default settings are fine for almost anyone, you should be aware that they exist, just in case.
First, you can prevent a particular app from using iCloud Drive (for added security or to save space):
Mac: Go to System Preferences > iCloud and click Options next to iCloud Drive. Then uncheck any app’s name and click Done. This hides that app’s folder in iCloud Drive on this Mac, but does not affect the data already saved to iCloud Drive, or its appearance on other devices. This setting doesn’t prevent you from manually selecting iCloud Drive as a destination for unchecked apps.
Note that one of the items you can select or deselect is Desktop & Documents Folders, but that requires its own discussion; see Sync Your Desktop and Documents Folders (or Don’t), ahead.
iOS: Go to Settings > Your Name > iCloud > iCloud Drive and, in the list below the iCloud Drive switch, turn off any app you want to disable on that device. That app loses in-app access to documents stored in iCloud Drive until you turn it back on. In addition, that app’s folder will no longer appear in the iCloud Drive document picker (for any app) on that device. You can, however, still access that app’s documents from the Files app.
Second, on a Mac you can select or deselect the Optimize Mac Storage checkbox. That also requires a bit of explanation; see Optimize Mac Storage.
Third, you can choose which apps let other people find you by your email address for the purpose of sharing documents (although truth be told, I haven’t encountered any apps that support this feature yet). To do this on a Mac, go to System Preferences > iCloud, click Options next to iCloud Drive, and click Look Me Up by Email. On an iOS device, go to Settings > Your Name > iCloud> Look Me Up.
Sync Your Desktop and Documents Folders (or Don’t)
A significant iCloud feature introduced in 10.12 Sierra was the option to sync your Desktop (~/Desktop) and Documents (~/Documents) folders using iCloud Drive. The idea sounds both simple and elegant: merely check a box, and everything in those two folders—which is to say, the vast majority of files (apart from photos and music) for most users—becomes available on all your Macs, plus on your iOS devices and on the iCloud website. Add a file on one Mac, and it appears on the others; change a file in one place, and it changes on the rest. You’ll never be without your important data, and never have to take manual steps to sync things among your devices. In other words, this feature is supposed to do for your files what iCloud’s other features already do for your email, contacts, calendars, notes, keychains, and so on.
In a moment, I’ll tell you how to turn this feature on (or off), and how to achieve a similar result without using iCloud if that’s your preference. But first I want to offer a reality check. I don’t use this feature myself, and I think it will prove problematic for lots of Mac users. Before you jump in (if you haven’t already), I want you to be fully aware of how it works and what the consequences will be.
How Desktop & Documents Folder Syncing Works
When you turn on syncing for your Desktop and Documents folders, macOS moves those two folders from their original locations to your iCloud Drive folder. (In most cases, the contents of any given Mac’s Desktop folder actually move into a subfolder of the new Desktop folder in iCloud Drive called “Desktop - Mac Name,” and likewise the contents of the Documents folder goes into “Documents - Mac Name.” In other words, upon initially activating this feature, the Desktop folders and Documents folders from your various Macs don’t merge with each other, but you can merge the files manually if you like—see Enable Desktop & Documents Folder Syncing for details.) Once it has done so, those folders, like everything else in iCloud Drive, begin syncing to Apple’s servers, and from there, to your other devices.
Let me be clear on what that means:
If you do not already have enough storage available in iCloud Drive for all that new data, you’ll be prompted to upgrade to a monthly plan with more capacity. (And, since the highest tier for iCloud storage is now 2 TB, you can’t use this feature if those two folders together, across your Macs that have the feature enabled, contain more data than that.) Depending on how much data you have, this can significantly increase the cost of using your Mac.
Even if storage limits are not a concern, you’ll have to wait for all that stuff to upload (and then download to your other devices). Depending on the amount of data you have and the speed of your broadband connection, this could take days, weeks, or even months. (And, if your ISP imposes a monthly data cap, as many do, using this feature could quickly use up your monthly data allocation, as a result of which you may be charged more, or have your service slowed down or limited in some way.) However, note that the complete files sync only to other Macs. Your iOS devices will show the files uploaded from your Mac, but they won’t be stored locally until you need to work with them.
If you have any huge files in your Documents folder—I’m thinking, for example, of virtual machines created by Parallels Desktop, VMware Fusion, or VirtualBox, which routinely run into the tens of gigabytes—those will pose a particular problem, because every time the data within a virtual machine changes (which might be many times a day), the entire virtual machine will have to be uploaded (and thus downloaded to other Macs) again. The net result could be that the files are perpetually out of sync. (If you have such files but still want to use this feature, I strongly recommend moving them out of your Documents folder before you enable syncing.)
The same goes for databases and other document repositories, such as those used by DEVONthink and Scrivener.
Although iCloud Drive encrypts your data in transit as well as on Apple’s servers, this protects it only from malicious outside parties. Nothing prevents Apple from accessing your data, and if a government agency demanded access to it, Apple would be both obligated and able to provide it.
If you also select the Optimize Mac Storage checkbox (more on this in a moment), macOS can delete the local copies of any of your documents, leaving only their icons behind (with a badge to indicate that the originals are stored in iCloud Drive and, presumably, on your other Macs), if your Mac starts running low on disk space.
The benefit of the Optimize Mac Storage setting is that if you have one Mac with plenty of storage but another with much less room, you won’t get stuck—both Macs will show all your files and folders, but on the lower-capacity Mac, some files won’t be stored locally, instead downloading only when you need them. However, if the moment you need them is one when you have no internet access (or a slow connection), you could be up a creek. And remember that any backup software you use (including Time Machine) can back up only those files for which there is a local copy. If something is stored solely on Apple’s servers, it won’t be included in your backups.
With Desktop & Documents syncing enabled, you will no longer have folders located at
~/Desktopor~/Documents. macOS transparently remaps those folders in the background, so apps, scripts, and other tools that are hard-coded to look in those locations should continue to work.You can’t choose to sync just your Desktop folder or just your Documents folder, or to sync any additional folders. It’s those two or nothing.
If you decide to turn off Desktop & Documents syncing, macOS will not put everything back where it was! It will recreate your Desktop and Documents folders in their original locations, but won’t move the data from iCloud Drive to those folders—you’ll have to do that manually.
You may get the idea that I’m a bit less than fully enthusiastic about this feature. The fact is, I think it could be fantastic for a certain type of user—namely, someone who has multiple Macs, whose Documents folder is relatively small (and with relatively small files inside it), and who isn’t especially concerned about the privacy of any of that data. If you fit that description, and the eight bullet points above don’t put you off, then by all means, use this feature with my blessing. If not, simply leave it disabled and carry on as if it never existed. (But, read Sync Desktop & Documents Folders without iCloud Drive for what I consider a better path to essentially the same end result.)
Enable Desktop & Documents Folder Syncing
If your Mac didn’t come with Sierra or later preinstalled, then in the process of upgrading to the latest version of macOS, the installer will have invited you to turn this feature on. (The relevant installer screen is titled “All your files in iCloud,” and the single checkbox “Store files from Documents and Desktop in iCloud Drive” is selected by default.) If you left that box checked, the feature is already on.
If you (wisely) deselected it during the upgrade and want to turn it on now, you can do so in either of two ways:
Choose Apple > About This Mac > Storage > Manage (or, if you prefer, open System Information in
/Applications/Utilitiesand choose Window > Storage Management, which opens the same window). Click the Store in iCloud button, make sure Desktop and Documents is selected if you’re given the choice, and click Store in iCloud. (Before enabling Photos in the same dialog, I suggest that you read Use iCloud Photo Library.)Open System Preferences > iCloud, click the Options button next to iCloud Drive, select the Desktop & Documents Folders checkbox, and click Done.
The change is immediate, although syncing to Apple’s servers may take quite a while.
As I mentioned earlier, if you use this feature on two or more Macs, iCloud moves those folders to Apple’s servers but does not merge them across your computers by default. You’ll have to do that manually, and thereafter, the files will sync as you expect. To perform this manual, one-time merge:
Select iCloud Drive in the sidebar of a Finder window, and inside that, select one of the two “Desktop - Mac Name” folders.
Drag the stuff that’s in that folder to your actual Desktop.
Repeat the above with the second “Desktop - Mac Name” folder, but if there are duplicate files or folders within the two folders already, you’ll have to sort those out manually.
Now delete the two empty “Desktop - Mac Name” folders from iCloud Drive.
Repeat steps 1–4 with the two Downloads folders.
Disable Desktop & Documents Folder Syncing
To disable this feature, open System Preferences > iCloud, click the Options button next to iCloud Drive, deselect the Desktop & Documents Folders checkbox, click Turn Off in the alert that appears, and click Done. Then click OK in the second alert confirming that the change has occurred.
Once you’ve done this, the items you’d stored in iCloud Drive will still be there, and your local copies will disappear—which may sound like the opposite of what you were expecting. So you’ll almost certainly want to move the files and folders from iCloud Drive back to your local Desktop and Documents folders. You can do so in the usual way (drag and drop); after dragging the files, click Move in the alert that asks if you really want to move them out of iCloud Drive. (For more on working with iCloud Drive in the Finder, see Explore iCloud Drive, ahead.) Note that the download may take quite a while, depending on the quantity of data you’ve stored in those two folders and the speed of your broadband connection.
Sync Desktop & Documents Folders without iCloud Drive
Apart from the issues I identified earlier with iCloud Drive’s syncing of Desktop and Documents folders, my initial reaction was, “This is a poor solution to a problem I solved years ago.” If you’re concerned about using this iCloud feature but wish you could have its benefits, let me briefly share my approach.
I use an app called Resilio Sync Home (formerly known as BitTorrent Sync), which lets me sync any number of folders on my Mac with other devices I own, or share them with other people. (I use it only for my own devices, however.) The syncing uses the same underlying peer-to-peer technology as BitTorrent, which means that it does not rely on a central server, like iCloud Drive. Devices talk directly to each other, over an encrypted connection.
I can use this tool to sync folders across computers (Mac or PC), and I can access all my files from an iOS device. I can’t reach them from the web (because, remember, they’re not stored on a server), but that’s fine by me—I don’t have to pay anyone for storage or worry about file sizes. Changes are synced almost instantly, and I can opt to save old and deleted versions of each file (à la Dropbox). And, for files over 4 MB that are modified, only the changed portions are transferred during a sync, which saves lots of time and bandwidth.
Resilio Sync Home comes in both free and Pro (paid) versions. The Pro version costs $59.99, and that’s what I recommend—primarily because it lets you link all your devices, such that sharing a folder on one automatically syncs it to all of them. The Pro version also offers Selective Sync, which approximates iCloud Drive’s Optimized Storage feature. If you enable Selective Sync for a folder, you’ll see its contents on your other devices but the files themselves won’t transfer until you try to open them (or explicitly request that they download). Note that with Selective Sync enabled, you won’t see thumbnails of graphics that haven’t been downloaded to your computer.
Optimize Mac Storage
If you go to System Preferences > iCloud and click the Options button next to iCloud Drive, you’ll notice an Optimize Mac Storage checkbox at the bottom of the dialog. When this is selected (as it is by default), your Mac keeps a local copy of everything in your iCloud Drive folder—including your Desktop and Documents folders, if you enabled syncing for them—unless you start running low on disk space. If you do, iCloud Drive automatically deletes older documents from your Mac, leaving only their icons (and other metadata) behind, while the originals remain stored in iCloud Drive. When you try to open a document whose content has been deleted from your Mac, macOS downloads it from iCloud Drive automatically. Other than taking a bit longer for such files to open, the process should be transparent.
If this sounds suspiciously like iCloud Music Library (see Use iCloud Music Library) or iCloud Photo Library (see Use iCloud Photo Library), that’s no coincidence—it’s Apple’s way of applying roughly the same logic to the rest of your iCloud documents. (But note that, despite the feature having both “optimize” and “storage” in its name, it’s merely a small subset of your Mac’s Optimized Storage capabilities, which I describe further in the sidebar About Optimized Storage.)
Optimized storage is great—and possibly indispensable—for users with very little free storage on their Macs. But it’s not risk-free. By enabling this feature, you’re trusting that your data won’t go missing from Apple’s servers, that there won’t be an iCloud outage (or problems with your internet connection) at the moment you need those files, and that macOS will make intelligent choices about which old files to remove from your Mac and when.
Personally, I feel safer having this box unchecked.
Explore iCloud Drive
Once you’ve activated iCloud Drive, you can use it almost like any other folder on a Mac. In the Mac Finder, iCloud Drive appears in the sidebar. (It’s near the top, under Favorites, if Desktop & Documents Folders is disabled. If it’s enabled, you’ll see iCloud Drive, Desktop, and Documents listed under an iCloud heading in the sidebar.) Select this (or choose Go > iCloud Drive) to open iCloud Drive (Figure 9).
You can drag files into or out of the iCloud Drive folder to move them to or from iCloud. You can also create folders and organize items within iCloud Drive almost exactly as you would in any other folder.
You may notice, however, that you don’t see a folder for every iCloud-enabled app you use (especially iOS apps), and some folders may not contain the files you think they should. (In fact, these items merely look like folders; they are actually called App Libraries, although I refer to them here as folders for simplicity.) I explain this, and other oddities, next.
Understand iCloud Drive Peculiarities
Remember how I said earlier that, with iCloud Drive, “you can (in theory) see any of the documents that any of your apps have stored in iCloud?” Well, theory and reality don’t exactly align, and the rules can be confusing. So, let me lay out the facts.
Apps Use iCloud in Different Ways
Every Mac app’s Open and Save dialogs show iCloud Drive in the sidebar (if you don’t see the sidebar in a Save dialog, click the arrow 
next to the filename field, or choose Other from the Where pop-up menu, to expand the dialog), which means you can navigate manually to any spot in iCloud Drive to open or save a document.
However, those with explicit support for iCloud Drive show an extra entry, bearing the app’s name, under iCloud at the top of the sidebar in that app’s Open and Save dialogs (Figure 10).
When you see an entry like this, it’s a shortcut to that app’s iCloud Drive folder. But individual app developers can choose to hide their app’s folder on one or more platforms, which means—depending on the app—that sidebar entry may be the only way to see that app’s documents. The folder may not show up in the Finder, on the iCloud website, and so on.
Developers can also determine which file types their app-specific folder may contain. So, if App X can’t open Pages documents, for example, it may not permit you to put a Pages document in its folder (even by dragging it in the Finder), and the folder for Pages won’t be available in that app’s document picker in iOS.
Opening Documents in Other iOS Apps Is a Bit Different
If you’ve used almost any app with built-in support for Dropbox, Google Drive, OneDrive, or similar services, you’re probably accustomed to the following behavior:
In the app, you select the cloud storage service you want to use, navigate to the file, and tap to open it.
You edit the file. Changes are saved automatically and immediately synced to your other devices.
If you open that file in another app, or on another device, it reflects the edits you made.
That’s simple, logical, and pretty much what anyone would expect. Previously, iCloud Drive behaved much differently, and as of iOS 12 and 10.14 Mojave, the above description is mostly what happens. But there are a few exceptions. For example:
Some apps let you open and edit their own native documents in place, even if they were stored outside the app’s folder. However, this behavior isn’t automatic, and a number of popular apps have not yet been updated to work this way. If you happen to use such an app to open one of its files stored in a different folder, it will first copy the file to its folder and then open the copy.
If a document is stored outside the folder of the app that created it, and it hasn’t yet been downloaded to your device when you try to open it, opening may fail. This is most likely a bug, and with any luck, Apple will address it soon.
If you open a document on an iOS device that is not in the app’s native format—for example, you open a plain text file from TextEdit in Pages—once again, the app typically copies the file, converts the copy to its native file type, and stores the new document in the same folder as the original. Any changes you make happen to the copy, not to the original.
Similarly, iOS apps generally don’t let you save their documents directly to another location in iCloud Drive, but you can sometimes export them, creating a copy (perhaps in a different format) in the new location.
Now that you have a better idea of what to expect, let’s look at a few examples of using iCloud Drive in other environments—within apps (on the Mac and in iOS), in the Files app for iOS, on the iCloud website, and on Windows.
Use iCloud Drive with Mac Apps
In the Finder, double-click a document anywhere in iCloud Drive to open it in its default app. Alternatively, in an app’s Open dialog, select the app’s name under iCloud in the sidebar (if present) to open documents located in its app-specific folder, or select iCloud Drive under Favorites to navigate to any file the app can open. (If you can’t open a file in the app, it appears dimmed and can’t be selected.)
iCloud-enabled apps typically store new files, by default, in their app-specific folder within iCloud Drive, but you can override this choice as you like, and can also move documents to a different location after the fact.
Use iCloud Drive within iOS Apps
As I explained earlier, iOS apps aren’t entirely consistent when it comes to iCloud Drive. So I want to offer some broad guidance and then give a few specific examples.
Somewhere within most iOS apps that support iCloud Drive is a document picker that looks something like Figure 11. (It also, not coincidentally, looks almost exactly like the Files app introduced in iOS 11, which I discuss next.)
This is not the main file view. For simplicity, most apps show you only their own documents by default—that is, what’s in the folder with that app’s name. (Remember, too, that some iOS apps use iCloud Drive exclusively for their own documents and provide no access to documents outside that folder.)
You may have to dig a bit to find iCloud Drive. For example:
In the latest versions of iWork apps (Pages, Numbers, and Keynote), the iCloud Drive document picker is the first thing you see, by default, when you open the app. It may, however, initially be set to show Recents (recently modified documents); to browse through all your files, tap Browse at the bottom. If you already have a document open, tap the word or icon in the upper-left corner of the screen to see the app’s documents; to navigate up to the top level of iCloud Drive, tap iCloud Drive in the upper-left corner.
In GoodReader, tap the Tools
icon and then tap Import (on an iPhone or iPod touch, you may have to swipe the control grid toward the left to see the Import button). Then tap either Import (Copy) from iCloud Drive or Open File from iCloud Drive.In Documents by Readdle, tap Documents > iCloud. Be aware, however, that this shows you only the app’s subfolder within iCloud Drive, not your entire iCloud Drive.
If you can’t find iCloud Drive, it may mean the app doesn’t fully support it—or the document picker might just be in an unexpected place. Check with the developer for guidance. (For more details on opening an app’s documents from another folder, see Opening Documents in Other iOS Apps Is a Bit Different.)
Use the Files App for iOS
For years, the only way iOS users could access iCloud Drive was from within supported apps (as I just described). Then, in iOS 9, Apple introduced a stand-alone iCloud Drive app for viewing and working with documents in iCloud Drive, which made the process more convenient and took iOS one big step closer to having a system-wide, user-accessible file system. In iOS 11, Apple took yet another big step, replacing the iCloud Drive app with an app Called Files.
Files, which is installed automatically with iOS 11 and later, looks and behaves much like the in-app iCloud Drive view I described a moment ago, except perhaps for the color scheme (Figure 12). It serves the same purpose, and can do all the same things, as the iCloud Drive app—but it goes beyond iCloud Drive in an important way.
What distinguishes the Files app from the iCloud Drive app is that it supports additional cloud service providers, as well as connections to numerous apps on your iOS device. So, just as you can navigate iCloud Drive and open, delete, or save documents there, you can do the same with a wide variety of cloud-connected apps (as long as they’re installed on your iOS device)—Dropbox, Google Drive, OneDrive, Transmit, Resilio Sync, DEVONthink To Go, and numerous others. There’s even an On My Device category for files stored locally on your iOS device and not in the cloud.
To view and navigate to these locations, start on the Browse screen and then tap Locations in the upper-left corner. (You may need to sign in the first time you use a new location.) To add new locations from supported apps on your iOS device, or disable existing locations, tap Edit (Figure 13) and then turn switches on or off; you can also drag the 
handle to reorder them.
Each location in the Files app has its own interface and features—so what you see when you tap Dropbox, for example, will look much different from what you see when you tap OneDrive. However, they should be similar enough to the stand-alone apps that you’ll be able to figure them out easily.
If you navigate to a file in the iCloud Drive portion of the Files app, here are some of the more interesting things you can do:
Open it: Tap the file to open it with the default app for that file type.
Rename it: Long-press the file, and tap Rename on the popover.
Delete it: Tap Select, select the file, and tap the Trash
icon or the word Delete.Copy it: Tap Select, select the file, tap the Folder
icon or the word Move, navigate to a new location, and tap Copy. Not all locations are valid destinations, and I have not yet been able to discern how Files decides where you can and can’t copy things. In addition, even though you may tap the word Move, I have not yet encountered a situation in which Files actually permits me to move a file.Share it: Tap Select, select the file, and tap the Share
icon. Then, on the Share sheet, tap Add People. Tap Share Options to specify public (“Anyone with the link”) or private (“Only people you invite”) access, and whether you’re sharing the file read-only (“View only”) or read-write (“Can make changes”). Tap a destination (such as Message or Mail), fill in the recipient(s), and send the message.
You can also use the Search field at the top to search for files anywhere on your iCloud Drive.
Use iCloud Drive on the Web
If you need access to your documents in iCloud Drive on a computer that isn’t signed in to your iCloud account (a friend’s computer, say, or a computer at a hotel or library)—or if you want to engage in real-time collaboration in an iWork app—you can do so by visiting the iCloud website in a browser (see The iCloud Drive Web App). You can open documents from Pages, Numbers, and Keynote (see The iWork Web Apps), but otherwise, you can open a file only if the computer you’re using has a compatible app.
Use iCloud Drive in Windows
After enabling iCloud Drive in the iCloud app (read Set Up iCloud under Windows), go to C:Usersyour-username and double-click iCloud Drive. There you’ll see all your iCloud Drive documents and folders, and you can interact with them just as with any other folder on your PC. But note that even though you can see documents created on a Mac or iOS device, you won’t be able to open them unless you have a Windows application that can read the appropriate file format.
Troubleshoot iCloud Drive
iCloud Drive has worked pretty much as expected in my testing so far, but I’ve read many reports of people who were less fortunate. If you run into trouble, here are a couple of tips that may help:
Syncing problems: If files aren’t syncing to or from your Mac as they should, confirm that you’re signed in to System Preferences > iCloud with the same Apple ID you use on your other devices.
Anything else: If you’re having a different problem—or if the above tip doesn’t work—it’s always worth trying The All-Purpose iCloud Troubleshooting Procedure I described earlier.
I should also mention a few Apple support articles on the web that may help you with iCloud Drive problems and questions:
Use In-App Data Syncing
Not all apps are based on documents, but even those that aren’t can use iCloud to synchronize other kinds of data. The easiest way to explain what I mean is by way of a few examples:
1Password can use iCloud (as an alternative to a 1Password.com account or Dropbox) to sync passwords, credit card numbers, and other private data with other copies of the app running in macOS or iOS.
Apple’s Podcasts app uses iCloud to synchronize custom stations and playback positions (so you can start listening to a podcast on one device and continue on another).
Tweetbot, a cross-platform Twitter client, can use iCloud to synchronize mute filters, read/unread status for direct messages, and timeline position across devices.
In addition, when iCloud Drive is enabled, iCloud syncs a few other bits of data across your devices, including text abbreviations (from System Preferences > Keyboard > Text on a Mac, or from Settings > General > Keyboard > Text Replacement on an iOS device) and several types of settings and data for Apple Mail: signatures, flag names (see this article for how to change the names), and (only on Macs) rules and smart mailboxes. (Although Mail rules sync, their enabled/disabled state does not, since you might want different rules enabled on each Mac.)
Developers must explicitly add iCloud syncing support to their apps, and not all apps are good candidates for this type of syncing. In general, iCloud syncing is turned off by default for third-party apps; you’ll have to look through each app’s preferences to see if it’s supported and how to turn it on. More often than not, it’s as simple as flipping a switch or choosing iCloud from a pop-up menu. On the other hand, Apple apps that use in-app syncing (such as Podcasts and Game Center) usually have iCloud syncing turned on automatically with no way to disable it.
But remember, iCloud syncing is for Macs and iOS devices only. If you use an app that also has a version for Windows, Linux, or Android, you may be better off with an alternative syncing method, such as Dropbox, if it’s offered.