Specifies the structure of a composite field index of the parent list.<35>

 <xs:complexType name="SPFieldIndex">
         <xs:element name="FieldRef" type="SPFieldIndexColumn"
                     minOccurs="2" maxOccurs="2" />
     <xs:attribute name="ID" type="core:UniqueIdentifierWithBraces"
                   use="required" />

This element MUST contain two elements of SPFieldIndexColumn type; the first one defines primary field of the index, and the second one defines the secondary field of the index. Composite field indices on three or more fields are not supported by this protocol.