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