The id attribute is the QIF id of the feature zone, used for referencing.
Source
<xs:element name="FeatureZoneAreaCircular" type="FeatureZoneAreaCircularType" substitutionGroup="FeatureZone"><xs:annotation><xs:documentation>The global FeatureZoneAreaCircularType element gives information about a circular area of the feature.</xs:documentation></xs:annotation></xs:element>