14887 Notes regarding the reification of topic map constructs as topic nodes. Every topic node reifies a subject. In a topic map graph, no two topic nodes are supposed to reify the same subject, although human error may permit this to happen, since machines cannot intuit subjects. All of the topic characteristics that are known about a given subject are supposed to be combined in the one topic node that corresponds to that subject.
Subject descriptors are information objects.
The above paragraph describes the domesticated sheep. The other subject is the subject descriptor itself, considered as an information object. For practical purposes, the reference is to that corresponding topic node. It's a rarity, but it's important for completeness. They simply provide references to resources.