## 4 An Example of Classification and Nomenclature  
The classification proposal refers to the GIS world. The scheme, which is based on the unambiguousness of the elements described, is essentially divided into two parts: a system consisting of \"layer-theme-class\" for the instances identification and a descriptive phase in coded form for the explicitation of their characteristics. As with the GIS world, it should be mandatory to implement a subset of data in each BIM model (corresponding to the GIS world national core). Moreover, other specific attributes and instances can be added to enrich the information of objects modeled. The \"class\" notion is the basic one in this structure. It defines a series of homogeneous objects in the building organization designed by UNI 8290, also characterized by similar mandatory information content. Each class is identified by a single name, which is a numeric code that combines the three hierarchical levels of layer, theme, and class. Working this backward, a series of classes constitutes a theme and a defined number of them composes a layer. The unambiguousness of the system becomes clear. Within a precise class, a series of attributes characterize each object. Some aspects are specific for a few types of objects contained in the BIM model of a particular building (General Identification--General ID). Others qualify it based on its role, type of material, and history. The building system breakdown reported in the UNI 8290 adapts well to this structure. With regard to its structures section, the classification method is proposed again by first introducing the numbering of the various levels. Below, there is an example of a pillar. First, a letter will introduce its name to specify the type of geometry (V for volumetric, S for surface, P for point). Therefore, its geometry will be a volume, defined by its closed side surface. The latter is composed (if the building exists) by the modeling software based on survey with the help of sections that divide it in a robust way from a computer point of view in the continuous part with the other structural elements with which it is consistent. Returning to the name, it will correspond to a sequence of three numbers, each consisting of two digits. As already mentioned, the numbers refer to the same levels of the structure. As far as the pillar is concerned, therefore, the code will be the following: V010201. The 01 indicates the belonging of the element to the structures layer, 02 corresponds to the elevation structures theme and, finally, 01 leads us to the specific vertical volumetric element Figure 6. The term \"vertical\" is maintained in order to avoid modifying in UNI, but it is not intended as a strictly geometric feature of the. It attempts to classify all those structural parts of the building that serve to raise it to height, without giving rise to walkable elements or that generate \"horizontal\" spaces. Precisely on this last observation, certain attributes will have to be defined, including those that differentiate the element from all the others in the same class. In the following part, a first provisional list of attributes to associate with the V010201 section is described:  
* ID_class: a unique number within the class (progressive or random). * Model-ID: a general identifier of the model to which the object belongs. It is possible to manage the model-ID uniquely at the territorial (national) level, and this makes the individual element exclusively defined at the country level, as it does for all GIS objects. * Type: one item of default and complete list of all objects that can be part of vertical elevation structures. A first set will contain the following types: pillar, wall, column, capital, plinth, lintel, arch...; then it will be available also the corresponding generic instances: not detected, not known, undefined. * Material: the attribute referred to the principal material which the element consists of (in the hypothesis of a first schematic approach).