The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:complexType name="GroupFeatureMeasurementType"><xs:annotation><xs:documentation>The GroupFeatureMeasurementType is the group feature measurement information for an individual group feature.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="ShapeFeatureMeasurementBaseType"/></xs:complexContent></xs:complexType>