The Test-FileCatalog command compares the content of the catalog file to the filesystem. Hashes are recalculated for each file.

If none of the content has changed, Test-FileCatalog will return Valid:

PS> Test-FileCatalog -Path C:TempACL -CatalogFilePath C:TempSecurityexample.cat
Valid 

If a file has been added, removed, or changed, the Test-FileCatalog command will return ValidationFailed.

At this point, the Detailed parameter can be used to see which file changed.

The command does not provide a summary of changes; instead, it returns all files and hashes from the catalog and all files and hashes from the path being tested:

PS> Set-Content C:TempACL33.txt –Value 'New content'
PS> $params = @{
>>     Path = 'C:TempACL'
>>     CatalogFilePath = 'C:TempSecurityexample.cat'
>>     Detailed        = $true
>> }
PS> Test-FileCatalog @params

Status        : ValidationFailed
HashAlgorithm : SHA1
CatalogItems  : {[11.txt, 3B88969F774811E6A5D634832BE099EDA42B5E72], ...}
PathItems     : {[11.txt, 3B88969F774811E6A5D634832BE099EDA42B5E72], ...}
Signature     : System.Management.Automation.Signature

These values can be used to find changes. First, assign the result of the command to a variable:

$params = @{
    Path            = 'C:TempACL'
    CatalogFilePath = 'C:TempSecurityexample.cat'
    Detailed        = $true
}
$result = Test-FileCatalog @params

Once done, files that have been added can be listed with the following code:

$result.PathItems.Keys | Where-Object { -not $result.CatalogItems.ContainsKey($_) } 

Files that have been removed are listed with the following code:

$result.CatalogItems.Keys | Where-Object { -not $result.PathItems.ContainsKey($_) } 

Files that have been modified are listed with the following code:

$result.PathItems.Keys | Where-Object { $result.CatalogItems[$_] -ne $result.PathItems[$_]} 

As the file catalog only stores hashes, the command is unable to describe exactly what has changed about a file, only that something has.