The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:complexType name="CurveFeatureItemBaseType" abstract="true"><xs:annotation><xs:documentation>The CurveFeatureItemBaseType is the abstract base type for shape feature items for features composed of curves.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="ShapeFeatureItemBaseType"/></xs:complexContent></xs:complexType>