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 a reference to a QIF id that may not include an AsmPath.</xs:documentation></xs:annotation></xs:element>
<xs:complexType name="ElementReferenceType"><xs:annotation><xs:documentation>The ElementReferenceType is an entity ID.</xs:documentation></xs:annotation><xs:sequence><xs:element name="Id" type="QIFReferenceType"><xs:annotation><xs:documentation>The Id element is a reference to a QIF id that may not include an AsmPath.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:complexType>