The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="WeldUCharacteristicMeasurementType"><xs:annotation><xs:documentation>The WeldUCharacteristicMeasurementType defines the results of a measured weld u characteristic evaluation.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="WeldGrooveCharacteristicMeasurementType"/></xs:complexContent></xs:complexType>