The id attribute is the QIF id of the feature zone, used for referencing.
Source
<xs:element name="FeatureZoneAreaIrregular" type="FeatureZoneAreaIrregularType" substitutionGroup="FeatureZone"><xs:annotation><xs:documentation>The global FeatureZoneIrregularArea element gives information about an irregular area of the feature.</xs:documentation></xs:annotation></xs:element>