In the documentation, it is stated that the keep flag allows to keep deleted files and directories during incremental backups.
I’ve tried to do some incremental backups with and without setting this flag after having deleted files, but did not see any differences.
Can someone explain me what exactly is the purpose of this flag ?
Sorry to bring this topic back up, but I’m still interested to have an answer about this flag. 
@jenche … My guess is this…
Monday Full backup
delete file x
Tues Incr Backup
Incremental backup can “keep track of deleted files/folders”. So if you restore from the incremental backup, it could have file x or not based on this setting.
If you look at the info when using “urbackupclient add-backupdir --help” it states to NOT use that tag. I’m guessing it’s used by devs to debug stuff and not to use in “live” setups.