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