Relationship rules are defined based on entity type. Remember, entities are defined before relationships. The Logical Modeler only allows meaningful relationships to be defined for a given entity type as the logical model can be built into a physical database. For example, a Parent/Child relationship can be defined only between Data, Variant and External entities.
These are the default relationship rules:
Also see 7.2 Relationship Rules.
Note: Abstractions are not entities. They are objects used to group entities. Abstraction cannot be used in relationships.