Introduction of the notion
Source: Semantic Aspect Guide; page 24
The class diagram is an indispensable part of object modeling. The diagram is not the model, but a partial, biased illustration of it. Once finished, the class model represents the whole substance of the system, both data and processes. (These two notions of data and processing do not belong to the semantic aspect but burgeon over IT terminology. In the semantic aspect, preference is given to the terms: information, action, transformations and associations of objects.)
Each diagram is realized with a communication goal in mind. It only presents the elements which contribute to this objective. Diagrams must be readable: size will be limited to a sheet of A4 paper and rules such as the famous magic number seven can be applied. This rule summarizes the work of G. Miller in experimental psychology: a good structure – from a presentation point of view – is made up of about seven elements (give or take two).
However, at least for the modeler or developer's needs, a class diagram can exceed these limits and even contain the whole of the model. It follows that this diagram will then be known as the class model. It will not necessarily be part of the file, but constitutes a tool to find one's way around the model.
UML diagram that shows a collection of declarative (static) UML model elements such as classes and types, with their contents and relationships (www.omg.org)