Union data types are those whose value spaces and lexical spaces are the union of value spaces and lexical spaces of two or more other data types. A union data type enables an element or attribute value to be one or more instances of a type drawn from the union or combination of multiple atomic and list types. Union types have a memberTypes attribute value that is a list of all the types in the union. Union types can also have pattern and enumeration facets.

Union data type elements may contain some attributes, which are described by http://www.w3.org/TR/xmlschema-1/#declare-datatype.

Refer to the XML Schema Part 0: Primer for more information on the union data type.