 The "Requirements" usually specifies the most important attributes of the requested system.In the "Design specification" documents, physical or software processes and systems are frequently the requested functions

For advertising and marketing of technical products, the number of functions they can perform is often counted and used for promotion.For example a calculator capable of the basic mathematical operations of addition, subtraction, multiplication, and division, would be called a "four-function" model; when other operations are added, for example for scientific, financial, or statistical calculations, advertisers speak of "57 scientific functions", etc.