xml:id 

<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>

Attribute Information

Used By

Attribute Group xml:specialAttrs

Source 

<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>

Sample