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