A group may be used to aggregate arbitrary CityObjects according to some user-defined criteria. Examples for groups are the buildings in a specific region, the result of a query, or objects put together for visualization purposes. Each group has a name (inherited from AbstractGMLType), functions (e.g., building group), a class and zero or more usages. A geometry may optionally be attached to a group, if the geometry of the whole group differs from the geometry of the parts. Each member of a group may be qualified by a role name, reflecting the role each CityObject plays in the context of the group. As subclass of _CityObject, a CityObjectGroup inherits all attributes and relations, in particular an id, names, external references, and generalization relations. As CityObjectGroup itself is a CityObject, it may also be contained by another group. Denotes the relation of a CityObjectGroup to its members, which are _CityObjects. The CityObjectGroupMemberType element must either carry a reference to a _CityObject object or contain a _CityObject object inline, but neither both nor none. Denotes the relation of a CityObjectGroup to its parent, which is a CityObject. The CityObjectGroupParentType element must either carry a reference to a _CityObject object or contain a _CityObject object inline, but neither both nor none. The parent association allows for modelling of a generic hierarchical grouping concept. Named aggregations of components (CityObjects) can be added to specific CityObjects considered as the parent object. The parent association links to the aggregate, while the parts are given by the group members.