The id attribute is the QIF id of the feature zone, used for referencing.
Source
<xs:element name="FeatureZoneCurveCircular" type="FeatureZoneCurveCircularType" substitutionGroup="FeatureZone"><xs:annotation><xs:documentation>The global FeatureZoneCurveCircular element gives information about a circle or arc on the feature.</xs:documentation></xs:annotation></xs:element>