sortKey

<xs:attribute name="sortKey">
  <xs:annotation>
    <xs:documentation>supplies the sort key for this element in an index, list or group which contains it.</xs:documentation>
  </xs:annotation>
  <xs:simpleType>
    <xs:restriction base="xs:token">
      <xs:pattern value="(\p{L}|\p{N}|\p{P}|\p{S})+"/>
    </xs:restriction>
  </xs:simpleType>
</xs:attribute>