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:element name="Id" type="QIFReferenceType"><xs:annotation><xs:documentation>The Id element is the QIF id of the measurement to be excluded.</xs:documentation></xs:annotation></xs:element>
<xs:element name="Reason" type="ExclusionReasonType"><xs:annotation><xs:documentation>Each Reason element gives the reason why this measurement is being excluded.</xs:documentation></xs:annotation></xs:element>
<xs:complexType name="ExclusionIdType"><xs:annotation><xs:documentation>The ExclusionIdType defines the QIF id of a measurement to be excluded from a statistical evaluation and the reason for that exclusion.</xs:documentation></xs:annotation><xs:sequence><xs:element name="Id" type="QIFReferenceType"><xs:annotation><xs:documentation>The Id element is the QIF id of the measurement to be excluded.</xs:documentation></xs:annotation></xs:element><xs:element name="Reason" type="ExclusionReasonType"><xs:annotation><xs:documentation>Each Reason element gives the reason why this measurement is being excluded.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:complexType>