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="PointSetReferenceBaseType" abstract="true"><xs:annotation><xs:documentation>The abstract PointSetReferenceBaseType is the base type for references to all or portions of point sets, either nominal or measured.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="QIFReferenceType"/></xs:complexContent></xs:complexType>