The meaning of requirements

We use the term requirements to denote what are often called functional requirements. Requirements are located in the environment, which is distinguished from the machine to be built. A requirement is a condition over phenomena of the environment. A specification is a restricted form of requirement, providing enough information for the implementer to build the machine (by programming it) without further environment knowledge. To describe requirements appropriately we must fit our descriptions into an appropriate structure. This structure must respect the distinction between the machine and the environment, and the distinction between those environment properties that are given (indicative descriptions) and those that must be achieved by the machine (optative descriptions). Formalisation is a fundamental problem of requirements engineering. Since most environments are parts of the physical world, and therefore informal, the formalisation task is inescapable. Some techniques are discussed for tackling this task. In particular, the use of designations is explained, and the distinction between definition and assertion. By using the smallest possible set of designated terms, augmented by appropriate definitions, the developer can create a narrow bridge between the environment and its descriptions in the requirements. In this way a sufficiently faithful approximation to the informal reality can be obtained.