Subject A can either be a concept that is a 'Topic', or in unexpected cases an entity outside of an ontology (or an ontology concept in a role of an entity being semantically annotated) that is a 'Topic' or is in the role of a 'Topic'. Object B can be any concept or entity outside of an ontology. In EDAM, 'is_topic_of' is not explicitly defined between EDAM concepts, only the inverse 'has_topic'.