Often the specification is not complete and not well well thought out. One of the pitfalls for the engineer is starting the design process before the specification is complete and reviewed with all the stockholders. All design specifications have in common that they capture the design in detail.
A commonly used format is the following.
- A summary of the purpose of the required product or process.
- Why do they need the product or process?
A summary of the preconditions.
- This can be the production cost price, the development time.
A summary of the functions.
- Functions are abstract concepts that describe a change of state.
- The function of a light bulb is too illuminate the environment.
A summary of the requirements.
- Requirements from physically measurable quantities.
- This can be the lifespan in years, the strength in N, the cost price in euros.
A list of wishes.
- Desires are usually not physical, non-measurable requirements. For example, that a product must be beautiful.
In complex products usually the so called V model as a design process is used. On the high level of the V model the specification can be limit in size. This will result in a manageable specification on each level of the V model. Each field of discipline or industry has his own sjabloon to define the specification.
For more info see: Functional specification: wikipedia