Let us turn now to the receiver of the communication, and consider the sort of actions that are taken with the design information that is received. These may be divided into two main classifications:
Evaluating actions, taken to assess the properties or merit of the design.
Generative actions that generate information from the model for use downstream of the design process, usually in order to progress its manufacture.
 In each case the actions involve the extraction of information from the design representation, and the combination of this with further information to form a new model.

Modeling using computer-aided design (CAD)
To support the design task, designers form a series of models using various representations of the design, and others involved in the evaluation of the design and in the manufacture of the product extract information from these models and, in the process, form new models to assist them in their work.

The Aim of CAD
To apply computers to both modeling and communication of designs.
There are two different approaches:
At a basic level, to use computers to automate or assist in such tasks as the production of drawings or diagrams and the generation of lists of parts in a design.
At more advanced level, to provide new techniques, which give the designer, enhanced facilities to assist in the design process.

Representation of Models
There are two types of models:
Models of form typically represented by drawings of components and their arrangement in assemblies.
Models of structure normally represented by diagrams that show the components of a system and how they are connected.

The representation of form using drawings
The technique of representing three-dimensional forms in two-dimensional space by means of engineering drawings -on paper or on a computer screen- is formally known as descriptive geometry.

The representation of structure using diagrams
In engineering diagrams the logical or physical structure of a system, in terms of the assembly of the primitive parts and the relationship between these, is shown by a series of symbols joined by connections. The rules for the symbols, and for the connections, are governed by conventions that have been established in standards.

Block Diagrams
At an early stage in the design process it may only be possible to define overall relationships between parts of a system, and a block diagram may be most appropriate. As a design is prepared for construction and manufacture, detailed wiring or piping diagrams are required.

Strengths and Weaknesses of Conventional Representations

Strengthens: serve engineers, communication.
Weaknesses: skill is required, it is possible to have conflicting models (errors), and complexity in the product may stretch the techniques to their limits, there is a risk of error misinterpretation when extraction of information from drawings and diagrams.