OptionalallowedAllowed types for Relationship.relatedObject. Same matching rules as allowedRelatingTypes.
OptionalallowedAllowed types for Relationship.relatingObject. A type matches when it equals one of these or any of its super-types does. Empty / missing means "any type allowed".
OptionalallowWhen false (default), self-references fire
RELATIONSHIP_SELF_REFERENCE_FORBIDDEN.
Optionallabel
Spec for one Relationship type.