From http://www.w3schools.com (Copyright Refsnes Data)

XML Schema union Element

Complete XML Schema Reference

Definition and Usage

The union element defines a simple type as a collection (union) of values from specified simple data types.

Element Information

Parent elements: simpleType

Syntax

<union
id=ID
memberTypes="list of QNames"
any attributes
>

(annotation?,(simpleType*))

</union>

(The ? sign declares that the element can occur zero or one time inside the union element)

Attribute	Description
id	Optional. Specifies a unique ID for the element
memberTypes	Optional. Specifies a list of built-in data types or simpleType elements defined in a schema
any attributes	Optional. Specifies any other attributes with non-schema namespace

Example 1

This example shows a simple type that is a union of two simple types:

<xs:element name="jeans_size">
  <xs:simpleType>
    <xs:union memberTypes="sizebyno sizebystring" />
  </xs:simpleType>
</xs:element>

<xs:simpleType name="sizebyno">
  <xs:restriction base="xs:positiveInteger">
    <xs:maxInclusive value="42"/>
  </xs:restriction>
</xs:simpleType>

<xs:simpleType name="sizebystring">
  <xs:restriction base="xs:string">
    <xs:enumeration value="small"/>
    <xs:enumeration value="medium"/>
    <xs:enumeration value="large"/>
  </xs:restriction>
</xs:simpleType>

Complete XML Schema Reference