Computer Aided Styling (CAS) is a digital design process primarily focused on creating visually appealing and detailed surfaces for design intent assessment. These surfaces are the ones that users can see and touch, making them crucial in product design, especially in industries like Automotive, Power-Sports, and Consumer Goods.

Computer Aided Styling (CAS) happens early in the product development stage, helping designers visualize and refine their ideas. It is therefore an iterative process happening before the final styling production model, commonly know as Class-A surfaces. At this stage, the focus is on blocking in the volume with enough fidelity so design contributors can react and make decisions.

While Autodesk Alias is still the most common software for industrial design shape development, new  approaches like polygonal modelling and sub-division surfacing have opened the doors to other tools like Blender, 3DS max, Maya, Modo, and Cinema 4D, which use to be more popular in the entertainment industry.

These approaches are gradually replacing the shape development that use to happen manually with Clay. Virtual Reality is also becoming increasingly popular for shape assessment, where needed corrections may be spotted earlier on and limit the cost of physical, hard modelled prototype iterations.

Although very fast, polygonal modelling don't interact well with NURBS based CAD softwares used by engineering. The process usually involves reconstruction of polygonal data in NURBS based CAD, so boolean operations may be performed with a solid part, i.e., for fastening feature integration.

Class-A surfacing refers to the creation of high-quality, production ready surfaces that are visible and touchable in a product. These surfaces are crucial in the aforementioned industries, where the visual and tactile quality of the product is paramount. Class-A surfacing involves a particular skillset which takes a lot of time to master. Ideally, it starts when the design maturity is close enough the final intent, and has been "frozen". 

Even if Class-A surfacing is more than often associated with Automotive exterior bodywork, there are other fields that use the same fundamentals. For instance Automotive interiors have lesser quality requirements than exterior, but do end up having more parts and interfaces, which is where the challenges lie. In addition to aesthetic requirements,  ergonomics and manufacturability are also part of the Class-A modeler's job.

In Power-Sports and Consumer Goods, Curvature Continuity (G2) is usually considered an acceptable quality level for surfaces to be designated as Class-A. The same goes for Automotive Interiors. Depending on the manufacturer, it is common to find "Curvature Tangency" or "Curvature Flow" (G3) in automotive exterior bodywork. Analysis tools like Curvature Combs, Zebra Stripe (Isophotes), Diagnostic Shaders, and Curvature Maps all help Class-A modellers identify inadequacies and surface quality problems before the 3D model goes to Computer Aided Machining (CAM) for fabrication.

In my book, Class-A has everything to do with the surface layout, how you build it, and the quality of the data you pass along for manufacturing – This is software-agnostic. Things like working with simple, single span curves and planarizing them as they get tweaked with curvature combs, using low-degree primary surfaces built to theoretical intersections, and using isoparametric curves whenever possible to build secondary surfaces – so alignment follows the natural CV flow of the layout – are all part of my workflow. Tertiary surfaces is where the experience comes in. The transition and merging of multiple surfaces into one, while keeping proper continuity and reflections, are where things often become challenging. Experience also dictates judgement calls on where to spend the efforts, and where to let go, so quality v.s. time is balanced.

The most commonly used tools by industry professionals are Autodesk Alias and ICEM Surf, although some manufacturers use Siemens NX, and 3DS CATIA Freestyle / Icem Design Experience. Still, more accessible CAD packages like Rhino and newcomer Plasticity can produce very high quality surfaces if proper tools are used. These tools give control over the surfaces' internal topology (surface degree / number of control vertices and segmentation) allowing seamless data transfer to engineering, and facilitating things like shells / offsets required for the creation of a solid part.