Sav file versus backup file

What’is the .sav file used for?

There is also a backup file.

What is the difference?

The difference is explained here: Automatic Backup (.sav and .bak) and Autosave - JabRef

It depends on the version of JabRef you are using. There was a big rework in JabRef 5.8

If you are using JabRef 5.9 and newer and you find .sav, .bak or .tmp files next to your library file in some folder, you could delete those, as backups are now stored in a special folder in the users profile directory.

Do I understand correctly that .sav, .bak or .tmp are NOT being used anymore?
Because I still have .sav files and I use jabref 5.9.
Maybe they are gone in 5.10?

They are still used, but automatically deleted.

Rough outline of what’s happening during a write to the .bib file:
A .tmp will be written → .bib copies to .sav.tmp copies to .bib.sav gets deleted → 20 seconds later, a copy of the .bib file will be stored as.bak file in the user data dir.

To my knowledge, JabRef will not automatically delete old existing .sav or .bak files outside the users profile directory, since, if this were the case, JabRef would potentially have deleted legitimate backups that were intentionally created. JabRef simply stopped creating those files outside of the users profile directory, unless you have found a bug. Even if you upgrade to a newer version of JabRef, you will have to remove those files manually, IF you want to remove them.