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Requirements

Requirements express a relationship between an actor and a System A requirement is a statement about the relationship between system and something in its environment. Requirement statements are constructed according to the pattern;

{Something in the environment} {affects the system} {in a particular way.}

Or conversely, {The system} {affects something in the environment} {in a particular way.} These may seem rather general but serve to show that requirements are always about how the environment and the system interact.

For example; The lawn mower cuts the grass to between 3 and 6 inches. This dictates a relationship between the mower and the grass where the mower is the subject system and the grass is in the environment.

Here is another example. The mower consumes not more than 1 gallon of gasoline per hour. This indicates that the subject system transforms some of its environment at a particular rate. This implies that gasoline is not part of the mower but part of its environment even though it is constrained by the gas tank which is part of the mower. So requirements define all the important relationships between what is outside of the system and the system itself. Another way to say it is, the requirements are the what the system should do, not how. Note The how is called design.

Categories

Requirements are divided into categories. The ones that most readily come to mind are called functional requirements. These are the requirements that are derived from system needs that say what the system is desired to do. Design constraints are a category of requirements that are imposed from some external agency. Then there is a set of requirements categories that need to be considered for all systems. These categories are referred to system abilities because all the categories end in the suffix -ability.

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