The GroupFeatureItemType is an individual group feature. A group feature is a complex feature defined by a set of features, some or all of which may themselves be group features.
The id attribute is the QIF id of the feature, used for referencing.
Source
<xs:complexType name="GroupFeatureItemType"><xs:annotation><xs:documentation>The GroupFeatureItemType is an individual group feature. A group feature is a complex feature defined by a set of features, some or all of which may themselves be group features.</xs:documentation></xs:annotation><xs:complexContent><xs:extension base="ShapeFeatureItemBaseType"/></xs:complexContent></xs:complexType>