For example, if you are planning to buy a new car for your family, you might make a list of the things that you need from the car. Your list might include the following features:
Less important features that you would like, such as a particular color, would be further down your list. At the end of the exercise, you have a list of user requirements, which specify the kind of car you want to buy.
The exercise of listing requirements for buying a car is fairly straightforward; however, the designers of that car need more.
The designers need system requirements, which describe the features the car must provide. From these they can prepare detailed design documents. Each part of the design must be tested; therefore, tests are specified in a separate document.
Well-defined requirements ensure that your customers get what they want and show you the type of product you have to build, or the type of service you have to provide. For information about writing requirements, see Get It Right the First Time, which is available on the welcome page of the Rational® DOORS® information center.