Type describing the thematic and geometric attributes and the associations of buildings. It is an abstract type, only its subclasses Building and BuildingPart can be instantiated. An _AbstractBuilding may consist of BuildingParts, which are again _AbstractBuildings by inheritance. Thus an aggregation hierarchy between _AbstractBuildings of arbitrary depth may be specified. In such an hierarchy, top elements are Buildings, while all other elements are BuildingParts. Each element of such a hierarchy may have all attributes and geometries of _AbstractBuildings. It must, however, be assured than no inconsistencies occur (for example, if the geometry of a Building does not correspond to the geometries of its parts, or if the roof type of a Building is saddle roof, while its parts have an hip roof). As subclass of _CityObject, an _AbstractBuilding inherits all attributes and relations, in particular an id, names, external references, and generalization relations. The name will be represented by gml:name (inherited from _GML) . list order for storeyHeightsAboveground: first floor, second floor,... list order for storeyHeightsBelowground: first floor below ground, second floor below ground,... The lodXMultiSurface must be used, if the geometry of a building is just a collection of surfaces bounding a solid, but not a topologically clean solid boundary necessary for GML3 solid boundaries. Intended function of a building. The values of this type are defined in the XML file BuildingFunctionType.xml, according to the dictionary concept of GML3. The values may be adopted from ALKIS, the german standard for cadastre modelling. If the cadastre models from other countries differ in the building functions, these values may be compiled in another codelist to be used with CityGML. Denotes the relation of an _AbstractBuilding to its building parts. The BuildingPartPropertyType element must either carry a reference to a BuildingPart object or contain a BuildingPart object inline, but neither both nor none.