Jabref meta data in bib-fiel / file format documentation (without the biblatex part)

Can I find a document about the JabRef 5 used biblatex file format somewhere?
Not the standard biblatex part of course.

I mean the groups meta-data etc.

And a very importan side question: How can I differentiate between a JabRef 4 and 5 bib-file? I see no version information in the file.

I only know this places with development documentation


But there is not the information I am looking for.