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