The id attribute is the QIF id of the feature zone, used for referencing.
Source
<xs:complexType name="FeatureZoneAreaIrregularType"><xs:annotation><xs:documentation>The FeatureZoneAreaIrregularType defines a feature zone as an irregular area defined by a poly-line boundary.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="FeatureZoneAreaBaseType"/></xs:complexContent></xs:complexType>