A union simple type specifies a union of atomic simple type values.

Example:

<xs:simpleType name="size">
   <xs:union memberTypes="xs:integer">
      <xs:simpleType>
         <xs:restriction base="xs:string">
           <xs:enumeration value="small"/>
           <xs:enumeration value="medium"/>
           <xs:enumeration value="large"/>
         </xs:restriction>
      </xs:simpleType>
   </xs:union>
</xs:simpleType>