The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:complexType name="OtherNonShapeFeatureItemType"><xs:annotation><xs:documentation>The OtherNonShapeFeatureItemType defines a non-shape feature item, usually associated with a non-dimensional characteristic.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="NonShapeFeatureItemBaseType"/></xs:complexContent></xs:complexType>