xml:id 

Regularizes the naming of an element and thus facilitates building links
between it and other resources. Each id attribute within a document must have a unique
value.

Attribute Information

Source 

<xs:attribute name="id" type="xs:ID">
  <xs:annotation>
    <xs:documentation>Regularizes the naming of an element and thus facilitates building links between it and other resources. Each id attribute within a document must have a unique value.</xs:documentation>
  </xs:annotation>
</xs:attribute>

Sample