The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:complexType name="OtherSurfaceFeatureDefinitionType"><xs:annotation><xs:documentation>The OtherSurfaceFeatureDefinitionType defines the other surface feature nominal information that can be common to one or more other surface features.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="SurfaceFeatureDefinitionBaseType"/></xs:complexContent></xs:complexType>