We present a brief introduction on how the functionality of a part is represented in its CAD model. In most cases, the goal of these research efforts had been the development of the representation itself; often the scope of the representation is very broad. In others it was very specific to a class of products where the design attributes and functionality are intimately coupled.
Dighe et al. [90,91] developed a system for a specific range of products (Injection molded product housing) where the basic functions are mounting and structural rigidity. Welch et al.  developed a system for sheet metal bracket design. The only functionality they needed to represent was load path---a task successfully accomplished in the context of their particular product domain. Schiebeler et al.  described a knowledge-based design assistant. This system represent functionality as a graph where the features are the nodes. The types of edges between the features depend on functional relation between the features.
Nielsen et al.  reported a system for iterative design where functionality is represented as the target values for different parameters. Thompson et al.  proposed a methodology for representing design rationale. Their design rationale may include plans constructed for planning future products and design constraints identified during the design process.
ElMaraghy et al.  proposed and implemented a design scheme based on functional features. The functions are pre-defined into the features in the library. Such functional features are also the core of work of Schulte et al. .
Sodhi and Turner argues that effective functionality representation can only be achieved at the assembly level of a product. They  present a state of the art survey of assembly modelling research which demonstrates some functional modelling. Gui et al. proposed  bond graph based system of assembly modeling from functional perspective.
Henderson et al. [100,101,102] developed of a system for conceptual modeling and representing functionality, features, dimensions and tolerances within a solid modeling system. Their functionality representation is based on textual descriptions that annotate the geometric model. This representation cannot directly be used for redesign purposes, as it does not lend itself to driving geometric queries and design modifications. The model described is detailed and may serve as a valuable guide for future development of functional models for other purposes.
There are other research works related to functionality, design history, design rationale representation, many of which are worth noting [103,104,105,106,107,108,109]. Detailed presentation of this body of work is beyond the scope of this paper.