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="PointSetReferenceRangeType"><xs:annotation><xs:documentation>The PointSetReferenceType defines a reference to a point set with a range of point indexes.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="PointSetReferenceBaseType"><xs:attribute name="range" type="Natural2Type" use="required"><xs:annotation><xs:documentation>The required range attribute is a pair of two indexes indicating the start and end (inclusive) of a range of points in a point set.</xs:documentation></xs:annotation></xs:attribute></xs:extension></xs:complexContent></xs:complexType>
<xs:attribute name="range" type="Natural2Type" use="required"><xs:annotation><xs:documentation>The required range attribute is a pair of two indexes indicating the start and end (inclusive) of a range of points in a point set.</xs:documentation></xs:annotation></xs:attribute>