BFO 2 Reference: For both terms and relational expressions in BFO, we distinguish between primitive and defined. ‘Entity’ is an example of one such primitive term. Primitive terms in a highest-level ontology such as BFO are terms that are so basic to our understanding of reality that there is no way of defining them in a non-circular fashion. For these, therefore, we can provide only elucidations, supplemented by examples and by axioms.
The http://purl.obolibary.org/obo/bfo/classes-only.owl variant of BFO ("bfo_classes_only.owl") includes only the class hierarchy and annotations from the full OWL version of BFO 2: http://purl.obolibary.org/obo/bfo.owl ("bfo.owl"). There are no object properties or logical axioms that use the object properties in bfo_classes_only.owl. As the logical axioms in the bfo_classes_only.owl variant are limited to subclass and disjoint assertions they are much weaker than the logical axioms in bfo.owl.
If you plan to use the relations that define BFO 2, you should import bfo.owl instead of bfo_classes_only.owl. To the extent that the relations are used without importing bfo.owl, be mindful that they should be used in a manner consistent with their use in bfo.owl. Otherwise if your ontology is imported by a another ontology that imports bfo.owl there may be inconsistencies.
See the BFO 2 release notes for further information about BFO 2. Please note that the current release of bfo.owl uses temporal relations when the subject or object is a continuant, a major change from BFO 1.