The id attribute is the QIF id of the characteristic, used for referencing.
Source
<xs:complexType name="SurfaceProfileNonUniformCharacteristicMeasurementType"><xs:annotation><xs:documentation>The SurfaceProfileNonUniformCharacteristicMeasurementType defines the results of a measured non-uniform surface profile characteristic evaluation.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="ProfileCharacteristicMeasurementBaseType"/></xs:complexContent></xs:complexType>