Errata exists for this version of the document.

The HasSubtype ReferenceType defines subtypes of types. Subtyping can only occur between Nodes of the same NodeClass. Rules for subtyping ReferenceTypes are described in 5.3.3.3. There is no common definition for subtyping DataTypes, as described in 5.8.3. The remainder of 6.3 specify subtyping rules for single inheritance on ObjectTypes and VariableTypes.