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