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