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="CharacteristicItemId" type="QIFReferenceType"><xs:annotation><xs:documentation>The CharacteristicItemId element is the QIF id of the characteristic item.</xs:documentation></xs:annotation></xs:element>
The EvaluateCharacteristicMeasurandType defines a measurand to measure a feature for evaluating a specified characteristic. A characteristic item provides both the characteristic and the feature item.
The id attribute is the QIF id of the measurand, used for referencing.
Source
<xs:complexType name="EvaluateCharacteristicMeasurandType"><xs:annotation><xs:documentation>The EvaluateCharacteristicMeasurandType defines a measurand to measure a feature for evaluating a specified characteristic. A characteristic item provides both the characteristic and the feature item.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="MeasurandBaseType"><xs:sequence><xs:element name="CharacteristicItemId" type="QIFReferenceType"><xs:annotation><xs:documentation>The CharacteristicItemId element is the QIF id of the characteristic item.</xs:documentation></xs:annotation></xs:element></xs:sequence></xs:extension></xs:complexContent></xs:complexType>