Functional Specification
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Abstract:

A functional specification (or sometimes functional specifications) is a formal document used to describe in detail for software developers a product's intended capabilities, appearance, and interactions with users. The functional specification is a kind of guideline and continuing reference point as the developers write the programming code. (At least one major product development group used a "Write the manual first" approach. Before the product existed, they wrote the user's guide for a word processing system, then declared that the user's guide was the functional specification. The developers were challenged to create a product that matched what the user's guide described.) Typically, the functional specification for an application program with a series of interactive windows and dialogs with a user would show the visual appearance of the user interface and describe each of the possible user input actions and the program response actions. A functional specification may also contain formal descriptions of user tasks, dependencies on other products, and usability criteria. Many companies have a guide for developers that describes what topics any product's functional specification should contain.