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