The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:element name="OtherNonShapeFeatureItem" type="OtherNonShapeFeatureItemType" substitutionGroup="FeatureItem"><xs:annotation><xs:documentation>The global OtherNonShapeFeatureItem element gives information about an other non-shape feature item.</xs:documentation></xs:annotation></xs:element>