Decision making occurs at every level of design. Consider two vehicles of different makes. They may both be reasonable cars with the same objective, yet their designs differ. The designers evaluate several elements and reach specific conclusions, resulting in an optimum design. A market survey gives an insight of what people want. Existing norms play a vital role. Once a major decision is made, the remaining design features will follow.

For example, once we’ve decided on the engine capacity, shape, and size, the rest of the design process will follow. A faulty judgment results in a bad design and product.
Design can be for a variety of products, and with our current expertise and knowledge bank, we have a lengthy list of design disciplines, such as ship design, building design, process design, bridge design, clothing/fashion design, and so on.

The emphasis here is on machine design. We now define a machine as a group of resisting bodies with successfully constrained relative motions that are utilized to convert other forms of energy into Mechanical energy or to transport and change available energy to perform productive work. A heat engine is defined as something that turns heat into mechanical energy. 

In many cases, however, the machines receive mechanical energy and use it to perform a specified duty, such as a hoist, a bicycle, or a hand-winch.
This modification or transformation of energy necessitates the use of a variety of machine elements, both tiny and large. Machine design is largely concerned with developing these pieces so that they may safely transmit forces and execute their functions. Consider the following simple mechanisms.
(a) A hand winch; (b) A small press with a power screw.
Each of these mechanisms produces some meaningful work through certain combinations of machine parts. Designing these mechanisms would include first designing the elements and then assembling them in order.