Supertype and subtype modeling is based on basic set theory and many of its uses and problems can be discerned by examining simple sets. A subtype entity defines a subset of the set that is defined by the supertype entity. Subtype entity proliferation in a model without defined goals for the entities leads to cluttered model diagrams that may hide relevant business detail from the analyst. A principal effect of a subtype entity is to constrain the cardinalities associated with a supertype’s attributes and outbound relationships. Optional attributes of the supertype can become required attributes of the subtype or optional outbound relationships can become required. The use of subtype modeling during a project carries implications for the data and application architectures that are chosen for the information systems implementation. A requirement often exists to be able to add additional subtypes or remove existing subtypes after the implementation of a system.