This question is currently being addressed in the context of
importing the RO Relation Ontology into BFO. For the moment, note
that the term 'relation' is ambiguous as between (at least):
1. the purely formal relations represented by the RO -- these are not
entities in the BFO sense, because they do not have instances
2. material relations such as John's love for Mary
3. material relational events such as John's kissing Mary
4. comparatives John's being taller than Mary
Only 2 and 3 are clear cases of entities in the BFO sense, and they
are specifically dependent continuants, and occurrents, respectively
BS