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="AsmPathId" type="QIFReferenceType"><xs:annotation><xs:documentation>The AsmPathId element is the identifier of the assembly path of this part/assembly.</xs:documentation></xs:annotation></xs:element>
<xs:element name="Centroid" type="PointSimpleType" minOccurs="0"><xs:annotation><xs:documentation>The optional Centroid element is the instance center of mass.</xs:documentation></xs:annotation></xs:element>
<xs:element name="Box" type="BoundingBoxAxisAlignedType" minOccurs="0"><xs:annotation><xs:documentation>The optional Box element is the instance bounding box.</xs:documentation></xs:annotation></xs:element>
<xs:complexType name="ValidationPartAssemblyInstanceType"><xs:annotation><xs:documentation>The ValidationPartAssemblyInstanceType defines a set of part/assembly instance validation properties.</xs:documentation></xs:annotation><xs:sequence><xs:element name="AsmPathId" type="QIFReferenceType"><xs:annotation><xs:documentation>The AsmPathId element is the identifier of the assembly path of this part/assembly.</xs:documentation></xs:annotation></xs:element><xs:element name="Centroid" type="PointSimpleType" minOccurs="0"><xs:annotation><xs:documentation>The optional Centroid element is the instance center of mass.</xs:documentation></xs:annotation></xs:element><xs:element name="Box" type="BoundingBoxAxisAlignedType" minOccurs="0"><xs:annotation><xs:documentation>The optional Box element is the instance bounding box.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:complexType>