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="PointSetReferenceSingleType"><xs:annotation><xs:documentation>The PointSetReferenceSingleType defines a reference to a single point within a point set but its index.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="PointSetReferenceBaseType"><xs:attribute name="index" type="NaturalType" use="required"><xs:annotation><xs:documentation>The required index attribute is the index of a single point in a point set.</xs:documentation></xs:annotation></xs:attribute></xs:extension></xs:complexContent></xs:complexType>
<xs:attribute name="index" type="NaturalType" use="required"><xs:annotation><xs:documentation>The required index attribute is the index of a single point in a point set.</xs:documentation></xs:annotation></xs:attribute>