The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="LineProfileCharacteristicItemType"><xs:annotation><xs:documentation>The LineProfileCharacteristicItemType defines a profile of a line characteristic item.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="ProfileCharacteristicItemBaseType"/></xs:complexContent></xs:complexType>