HasSubtype References are used to define subtypes of ReferenceTypes. It is not required to provide the HasSubtype Reference for the supertype, but it is required that the subtype provides the inverse Reference to its supertype. The following rules for subtyping apply.

1. The semantic of a ReferenceType (e.g. “spans a hierarchy”) is inherited to its subtypes and can be refined there (e.g. “spans a special hierarchy”). The DisplayName, and also the InverseName for non-symmetric ReferenceTypes, reflect the specialization.
2. If a ReferenceType specifies some constraints (e.g. “allow no loops”) this is inherited and can only be refined (e.g. inheriting “no loops” could be refined as “shall be a tree – only one parent”) but not lowered (e.g. “allow loops”).
3. The constraints concerning which NodeClasses can be referenced are also inherited and can only be further restricted. That is, if a ReferenceType “A” is not allowed to relate an Object with an ObjectType, this is also true for its subtypes.
4. A ReferenceType shall have exactly one supertype, except for the References ReferenceType defined in 7.2 as the root type of the ReferenceType hierarchy. The ReferenceType hierarchy does not support multiple inheritances.