Relationship between two entity types where the following applies:
The target entity type depends on the source entity type for its
existence - that is, an instance of the target entity type can last no longer than the instance of the source entity type.
The target entity type is created from the source entity type - that
is, the source entity type affects the attribute instance directly.
When the target entity type is formed, there must be at least one
additional source entity type that differs from the first source entity type.