The IDREF data type is derived from the NCName type. : IDREF « XML Schema « XML Tutorial

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3. IDREF

IDREF refers to an element or attribute of type ID in the same instance document.
The value for IDREF must match a value for ID in the same instance document. 


<!-- schema -->
<xsd:element name="bank">
  <xsd:complexType>
   <xsd:complexContent>
    <xsd:extension base="xsd:anyType">
    <xsd:attribute name="branch" type="xsd:ID"/>
    </xsd:extension>
   </xsd:complexContent>
  </xsd:complexType>
</xsd:element>
<xsd:element name="city">
  <xsd:complexType>
   <xsd:complexContent>
    <xsd:extension base="xsd:anyType">
    <xsd:attribute name="branch_division"
     type="xsd:IDREF"/>
    </xsd:extension>
   </xsd:complexContent>
  </xsd:complexType>
</xsd:element>
<!-- instance document -->
<bank branch="412x">Northern</bank>
<city branch_division="412x">Chicago</city>