The Automatic Quit feature will shut down Scrivener automatically after a certain period of inactivity. This is especially useful for preventing conflicts and version issues if you often walk away, forget to close Scrivener, and then try to work on a project on another device.
The default inactivity period is set to one hour. You can adjust the time period and turn the feature on/off under Scrivener>Preferences>General>Automatic Quit.
When Scrivener shuts down automatically, it will backup (unless the project is excluded from backups) and close all open projects. When you launch Scrivener again, those same projects will reopen.