This option can be found under the Clean dropdown list from application's toolbar.
Using this option, Backup4all will delete from the backup destination all files which does not belong to the backup job.
How it works?
Backup4all will scan the destination folder against the information from the backup catalog. All files which does not exist in the backup catalog, will be deleted from destination.
This option can be used for Mirror backup jobs, if for some reasons there are files deleted from sources which could not be removed from destination at that time.
Or if you manually copied by mistake some files in the backup destination files.
When you use this option, a warning message is shown, also informing you about the risks: if there are errors in the catalog this might delete files you might need.