An Introduction To Obo Ontologies / quote 1 of 2
Class-level and instance-level relations
Relations (also called "relationship types" in this guide, or "Properties" in other description logic languages) model types of relationships between entities.
Any given relation can be applied in two ways, as "class level" relations, or as "instance level" relations. Class level relations relate two classes, and instance level relations relate two instances. For every class level application of a relation, there is at least one possible instance-level application of that relation.
Class-level relations can be thought of as describing required instance-level relationships that must exist when a class is instantiated. For example, consider the class-level relationship finger part_of hand. This means that for any particular finger in the real world (e.g., John's left index finger) there must be a particular hand in the real world (John's left hand) that the finger is part of. That relationship between particulars (johns_left_index_finger part_of johns_left_hand) is an instance-level relationship.