There are several different types of CAD,^[9] each requiring the operator to think differently about how to use them and design their virtual components in a different manner. Virtually all of CAD tools rely on constraint concepts that are used to define geometric or non-geometric elements of a model.

There are many producers of the lower-end 2D sketching systems, including a number of free and open-source programs. These provide an approach to the drawing process where scale and placement on the drawing sheet can easily be adjusted in the final draft as required, unlike in hand drafting.

3D wireframe is an extension of 2D drafting into a three-dimensional space. Each line has to be manually inserted into the drawing. The final product has no mass properties associated with it and cannot have features directly added to it, such as holes. The operator approaches these in a similar fashion to the 2D systems, although many 3D systems allow using the wireframe model to make the final engineering drawing views.

3D “dumb” solids are created in a way analogous to manipulations of real-world objects. Basic three-dimensional geometric forms (e.g., prisms, cylinders, spheres, or rectangles) have solid volumes added or subtracted from them as if assembling or cutting real-world objects. Two-dimensional projected views can easily be generated from the models. Basic 3D solids do not usually include tools to easily allow the motion of the components, set their limits to their motion, or identify interference between components.

There are several types of 3D solid modeling

• Parametric modeling allows the operator to use what is referred to as “design intent”. The objects and features are created modifiable. Any future modifications can be made by changing on how the original part was created. If a feature was intended to be located from the center of the part, the operator should locate it from the center of the model. The feature could be located using any geometric object already available in the part, but this random placement would defeat the design intent. If the operator designs the part as it functions, the parametric modeler is able to make changes to the part while maintaining geometric and functional relationships.
• Direct or explicit modeling provide the ability to edit geometry without a history tree. With direct modeling, once a sketch is used to create geometry it is incorporated into the new geometry, and the designer only has to modify the geometry afterward without needing the original sketch. As with parametric modeling, direct modeling has the ability to include the relationships between selected geometry (e.g., tangency, concentricity).
• Assembly modelling is a process which incorporates results of the previous single-part modelling into a final product containing several parts. Assemblies can be hierarchical, depending on the specific CAD software vendor, and highly complex models can be achieved (e.g. in building engineering by using computer-aided architectural design software)^[10]: 539

Top-end CAD systems offer the capability to incorporate more organic, aesthetic and ergonomic features into the designs. Freeform surface modeling is often combined with solids to allow the designer to create products that fit the human form and visual requirements as well as they interface with the machine.

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