id (as an attribute name) 

 denotes an attribute whose value
 should be interpreted as if declared to be of type ID.
 This name is reserved by virtue of its definition in the
 xml:id specification.


See http://www.w3.org/TR/xml-id/
for information about this attribute.

<xs:attribute name="id" type="xs:ID">
    <xs:annotation>
        <xs:documentation>
            <div>
                <h3>id (as an attribute name)</h3>
                <p>
       denotes an attribute whose value
       should be interpreted as if declared to be of type ID.
       This name is reserved by virtue of its definition in the
       xml:id specification.</p>
                <p>
      See <a href="http://www.w3.org/TR/xml-id/">http://www.w3.org/TR/xml-id/</a>
      for information about this attribute.
     </p>
            </div>
        </xs:documentation>
    </xs:annotation>
</xs:attribute>