The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:complexType name="OtherCurveFeatureDefinitionType"><xs:annotation><xs:documentation>The OtherCurveFeatureDefinitionType defines the other curve feature nominal information that can be common to one or more other curve features.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="CurveFeatureDefinitionBaseType"/></xs:complexContent></xs:complexType>