The optional xId attribute is a reference to the id of a QIF object in an external document. A QIF object in an external QIF document can be referenced by using references to two QIF ids: the id of the external document reference of type ExternalQIFDocumentReferenceType found in the local document and the id of the object found in the external document.
Source
<xs:complexType name="PointSetReferenceWholeType"><xs:annotation><xs:documentation>The PointSetReferenceWholeType defines a reference to a whole point set.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="PointSetReferenceBaseType"/></xs:complexContent></xs:complexType>