The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="ThicknessCharacteristicDefinitionType"><xs:annotation><xs:documentation>The ThicknessCharacteristicDefinitionType defines information that can be common to more than one thickness characteristic.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="LinearCharacteristicDefinitionBaseType"/></xs:complexContent></xs:complexType>