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