When you have many entries in your bibtex file, it
becomes difficult to distinguish to which research area a
particular entry belongs (even with keywords). It would
be nice to have an ontology with linked concepts (like
data storage -> databases -> database theory ). Then
you should be able to create a link from a bibtex entry
to such a concept to indicate that it deals for example
with database theory. The advantage would be that
when you are looking for entries in a particular sub-area,
you could navigate through the ontology graph like
through a mindmap and eventually get to your PDF.
Related to: https://sourceforge.net/p/jabref/feature-requests/831/
and https://sourceforge.net/p/jabref/feature-requests/632/
First part ("ontology with linked concepts") seems like the group feature