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