If, after serious consideration of the customer's enquiry specification, a contractor decides to prepare a tender, the contractor must obviously develop technical and commercial proposals for carrying out the work. These proposals will also provide a basis for the contractor's own provisional design specification. It is usually necessary to translate the requirements defined by the customer's specification into a form compatible with the contractor's own normal practice, quality standards, technical methods and capabilities. The design specification will provide this link.

It is well known that there are often several different design approaches (proposed solutions) by which the desired end results of a project can be achieved. There are therefore likely to be considerable technical differences between the proposals submitted by different companies competing for the same order. Since the contract has been awarded, however, the number of possible design solutions should have been reduced to one: namely, that put forward by the chosen contractor. But within that general solution or concept there might still exist a considerable range of different possibilities for the detailed design and make-up of the project.

Taking just a tiny element of a technical project as an example, suppose that an automated plant is being designed in which there is a requirement to position a lever from time to time by automatic remote control. Any one or combination of a number of drive mechanisms might be chosen to move the lever. Possibilities include hydraulic, mechanical, pneumatic, or electromagnetic devices. Each of these possibilities could be subdivided into a further series of techniques. If, for example, an electromagnetic system were chosen, this might be a solenoid, a stepping motor or a servomotor. There are still further possible variations within each of these methods. The device chosen might have to be flameproof of magnetically shielded, or special in some other respect.

Every time the lever has been moved to a new position, several methods can be imagined for measuring and checking the result, including electro-optical, electrical, electronic, or mechanical. Very probably the data obtained from this positional measurement would be used in some sort of control or feedback systems to correct errors. There would, in fact, exist a very large number of permutations of all the possible ways of providing drive, measurement, and positional control. The arrangement eventually chosen might depend not so much on the optimum solution (if such exists) as on the contractor's usual design practice or simply on the personal preference of the engineer responsible.

Without detailed design specifications, there would be a danger that a project could be priced and sold against one set of designs solutions, but actually executed using a different, more costly, approach. This danger is very real. The danger is greater when the period between submitting a quotation and actually receiving the order exceeds a few weeks, allowing time for the original intentions to be forgotten. Thus, it is important to develop quality design specifications when the project is first proposed and be able to stick close to those specifications through the life cycle of the project.