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

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3. IDREFS

The value space is the set of finite-, nonzero-length sequences of IDREF values. 
The lexical space is the white-space-separated list of IDREF values.

<!-- schema -->
<xsd:complexType name="bankref">
  <xsd:sequence>
   <xsd:element name="bank" minOccurs="0"
    maxOccurs="unbounded">
    <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:sequence>
</xsd:complexType>
<xsd:element name="city">
  <xsd:complexType>
   <xsd:complexContent>
    <xsd:extension base="xsd:anyType">
     <xsd:attribute name="branch_division"
      type="xsd:IDREFS"/>
    </xsd:extension>
   </xsd:complexContent>
  </xsd:complexType>
</xsd:element>

<!-- instance document -->
<bank branch="412x">Northern</bank>
<bank branch="513a">Central</bank>
<bank branch="793c">Southern</bank>
<city branch_division="412x 513a 793c">Chicago</city>