The HasSubtype ReferenceTypedefines subtypes of types. Subtyping can only occur between Nodesof the same NodeClass. Rules for subtyping ReferenceTypesare described in 5.3.3.3. There is no common definition for subtyping DataTypes, as described in 5.8.3. The remainder of 6.3specify subtyping rules for single inheritance on ObjectTypesand VariableTypes.