If the document's schema is an XML Schema [XML Schema Part 1] , this constant represents the union if simple types are involved.
The reference type definition is derived by union from the other type definition if there exists two type definitions T1 and T2 such as the reference type definition is derived from T1 by DERIVATION_RESTRICTION or DERIVATION_EXTENSION, T2 is derived from the other type definition by DERIVATION_RESTRICTION, T1 has {variety} union, and one of the {member type definitions} is T2. Note that T1 could be the same as the reference type definition, and T2 could be the same as the other type definition.