The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="WeldCharacteristicMeasurementBaseType" abstract="true"><xs:annotation><xs:documentation>The WeldCharacteristicMeasurementBaseType is the abstract base type that defines the results of a measured welding characteristic evaluation.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="CharacteristicMeasurementBaseType"/></xs:complexContent></xs:complexType>