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