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