The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="UserDefinedForceCharacteristicItemType"><xs:annotation><xs:documentation>The UserDefinedForceCharacteristicItemType is a characteristic item specified by the user that is measured in force units.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="ForceCharacteristicItemBaseType"/></xs:complexContent></xs:complexType>