What Types of Requirements Are There?

Requirements come in many types: size, weight, operation, safety, regulatory, and so forth. However, they can be put into two higher-level categories—functional requirements and nonfunctional requirement. Functional requirements are those that relate to the “functioning” of the system. For example, “The system shall display the company name, ticker symbol, current share price, and percentage change from yesterday's closing price on one line.” Functional requirements are the things that the system must “do.”

Nonfunctional requirements relate to the characteristics of the system as a whole: reliability, scalability, performance, mean time between failure, etc. For example, “The system shall operate at a 99.99% reliability 24 hours per day.” Nonfunctional requirements are the way the system as a whole must “be.”