IS-A Supertypes-Subtypes
A subtype entity is an entity that contains a set of optional attributes of the supertype entity and inherits its attributes and its relationships from the supertype entity
If the supertype entity is related to mutually exclusive subtype entities (can belong to only one subtype), indicate with 1; if subtypes are overlapping (can belong to more than one), use m
The same identifier (e.g., ClientID) is
used for the supertype as well as subtype.