The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="OtherFormCharacteristicNominalType"><xs:annotation><xs:documentation>The OtherFormCharacteristicNominalType defines a unique form characteristic.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="FormCharacteristicNominalBaseType"/></xs:complexContent></xs:complexType>