Functional specification

Typically, such consensus is reached after one or more reviews by the stakeholders on the project at hand after having negotiated a cost-effective way to achieve the requirements the software needs to fulfill.

In the ordered industrial software engineering life-cycle (waterfall model), functional specification describes what has to be implemented.

The next, Systems architecture document describes how the functions will be realized using a chosen software environment.

In non industrial, prototypical systems development, functional specifications are typically written after or as part of requirements analysis.

One popular method of writing a functional specification document involves drawing or rendering either simple wire frames or accurate, graphically designed UI screenshots.

Systems engineering model of Specification and Levels of Development. During system development a series of specifications are generated to describe the system at different levels of detail. These program unique specifications form the core of the configuration baselines. As shown here, in addition to referring to different levels within the system hierarchy, these baselines are defined at different phases of the design process.Note: There is one minor (and ironic) typo in the image above. SI&T is "System Integration and Test" not "System Integration and Text".