The concept of generic city objects and attributes allows for the storage and exchange of 3D objects which are not covered by any explicitly modelled thematic class within CityGML or which require attributes not represented in CityGML. These generic extensions to the CityGML data model are realised by the classes GenericCityObject and _genericAttribute defined within the thematic extension module Generics (cf. chapter 7). In order to avoid problems concerning semantic interoperability, generic extensions shall only be used if appropriate thematic classes or attributes are not provided by any other CityGML module