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