The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="GeometricCharacteristicNominalBaseType" abstract="true"><xs:annotation><xs:documentation>The GeometricCharacteristicNominalBaseType is the abstract base type that defines a geometric characteristic nominal.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="CharacteristicNominalBaseType"/></xs:complexContent></xs:complexType>