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