This directory serves multiple purposes, some of which may be not yet documented by Apple.
Untitled, Unsaved documents for supporting applications
Applications such as Preview and TextEdit.
Copy an image to your clipboard. Open Preview, close all windows, use the New from Clipboard command, quit.
Open TextEdit, close all windows, type something in a new window, quit.
In Terminal, run two commands:
ls -@l ~/Library/Containers/com.apple.Preview/Data/Library/Autosave\ Information
ls -@l ~/Library/Containers/com.apple.TextEdit/Data/Library/Autosave\ Information
Unsaved… files will be listed.
System-saved versions of user-saved files, where the file system or volume does not support permanent version storage
File systems such as MS-DOS (FAT); volumes mounted using AFP 3.3 or SMB; and so on.
Make an AFP or SMB connection to a file server. Use Preview to open an image on that server, edit that image, quit without saving a version, disconnect from the server.
In Terminal, run a command:
ls -lrt ~/Library/Containers/com.apple.Preview/Data/Library/Autosave\ Information
At the foot of the list will be a directory that contains the version automatically saved by the System.
In the same network environment: if you re-open Preview, the system may reconnect to the file server. Then in a version browser view of the remote file, the timeline will include a local file.
(I could go into greater detail, but it would be off-topic from the opening question.)
Beyond the two purposes noted above,
~/Library/Containers may have other purposes.
Allow the System to manage files in these areas.
Avoid dataloss; do not attempt unnecessary changes to
~/Library or its subdirectories.