Relationship: Category

Use

The category describes the relationship from the viewpoint of the target entity type. The category can be hierarchical, aggregating, referential, or external.

Hierarchical:

The target entity type is identified by the source entity type and is dependent for its existence on the source entity type. The key of the source entity type becomes part of the key of the target entity type. You may not change the relationship between two entities.

Aggregating:

The target entity type is identified by more than one source entity type and is dependent for its existence on the source entity types. The keys of the source entity types become part of the canonic key of the target entity type.

There are also special cases where the target entity type is not identified by one or more of the source entity types involved.

Referential:

The target entity type refers to the source entity type, that is, it is not identified by the source entity type. A relationship between two entities can be changed. You may not change the relationship between two entities.

External:

A relationship between an entity type within a data model and an entity type outside the data model is described as an 'external' relationship.