Dimensions are derived from the geometric objects, and act as attributes to them. Any changes would be difficult once these are entered in the systems because a lot of manual work is required for reconfiguration. However, in the model development cycle, some design changes are typically needed before the full requirements for functionality, construction and quality of design are met, which means only conceptual models are initially applicable, as in conceptual models, exact shapes and positions are not required and dimensions are always treated as parameters. These imply that there is a need for more flexible tools to support the conceptual model design in which the users need not worry about the final satisfactory design. The flexible tools should be capable of driving the specifications of geometric shapes by dimensions and geometric constraints to satisfy the conceptual design semantics.
Parametric design reflects the development of such intelligence.
It allows the designers to make modifications to existing designs by changing the parametric values, thus making it possible for them to create shapes without knowing precisely how they will be configured in the final design. However, existing researches in parametric design are mainly focused on 2D model. Fewer approaches can be found on 3D models. As 3D models have attracted more and more concerns in design area, it is very necessary to develop a 3D parametric model for better utility. Although this concept is not addressed in current STEP, it has to be considered seriously because this belongs to the near future major CAD requirements.