Let's first look at the ways that applications can handle multiple concurrent documents. These interfaces all aim to present a way to work with many documents at the same time. Whether it's a word processor, an image editor, or a web browser, there are many ways to approach this. An operating system typically has a default behavior that application developers are encouraged to use (sometimes by promoting enhanced usability with the latest changes, and other times by adding or removing APIs within their respective toolkits). These interface preferences can change over time, but can also become standardized around certain categories of application. For example, earlier in their history, Microsoft promoted the Windows multiple document interface (MDI) layout, which remains popular with text editors and integrated development environments (IDEs):

Developers of native macOS applications are encouraged to use a new window for each document, but to group them under the same application, so the user only sees one icon that groups them:

The Chrome web browser decided to integrate their tabbed display into the window header bar, a space that normally shows just the title of the application or loaded document:

With all these possible approaches, which is right for your application? If you have to deal with multiple documents, it's worth looking at applications that manage similar file types, or comparing how various applications in the same environment handle window management.