The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:element name="CircleFeatureDefinition" type="CircleFeatureDefinitionType" substitutionGroup="FeatureDefinition"><xs:annotation><xs:documentation>The global CircleFeatureDefinition element gives information about a circle feature that may be used by more than one feature.</xs:documentation></xs:annotation></xs:element>