The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="CoaxialityCharacteristicItemType"><xs:annotation><xs:documentation>The CoaxialityCharacteristicItemType defines a coaxiality characteristic item (ISO specific characteristic type).</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="LocationCharacteristicItemBaseType"/></xs:complexContent></xs:complexType>