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