A specialization category can be characterized by the following attributes:
Each entity of the generalization occurs at least once in a specialization of the category.
Each entity of the generalization occurs in a maximum of one specialization of the category.
The specialization category Activity is complete in the university example, since each university member belongs to at least one specialization. This specialization category is not disjoint, however, since it is conceivable that a person might be at the same time both a student and a research assistant with the result that they would occur in both specializations.
A specialization category need be neither complete nor disjoint.