The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="DepthCharacteristicMeasurementType"><xs:annotation><xs:documentation>The DepthCharacteristicMeasurementType defines the results of a measured depth characteristic evaluation.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="LinearCharacteristicMeasurementBaseType"/></xs:complexContent></xs:complexType>