The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:complexType name="OtherNonShapeFeatureMeasurementType"><xs:annotation><xs:documentation>The OtherNonShapeFeatureMeasurementType defines the measured values of an other non-shape feature.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="NonShapeFeatureMeasurementBaseType"/></xs:complexContent></xs:complexType>