In Section 2.1, we explained that models are homomorphic with their subject under study (SUS). A consequence of this is that models are not monolithic entities, but, on the contrary, are composed of individual entities that are organized in such a manner that they mimic entities of the SUS. Let us call the simplest entities that compose a model “model units”. A model unit is, therefore, an atomic component of a model, which represents a cohesive fragment of information in the SUS being modelled. Model units are the basic unit of homomorphism and can be classified into different types; for example, consider the architectural scale model described in Section 2.1. Discrete entities in this model (i.e. model units), such as those representing rooms, walls or stairs, can be mapped to entities in its SUS. A room entity in the model can be mapped to a room in the real building, and a wall entity in the model to a wall in the real building. Several room entities can exist in the model, each of them being mapped to a different room in the SUS. All the model room entities, however, can be said to belong to the same model unit kind, since all of them are room entities and can be described by their commonalities. A “model unit kind”, then, is a specific kind of model unit, characterized by the nature of the information that it represents and the intention of using such a representation. In an architectural scale model, we are likely to find model units of several kinds, ...

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