See: specification, requirements.

A formal document that captures the problem need and solution requirements of the new system. The spec must be written such that the requirements are testable, modifiable, traceable, concise, complete, non-ambiguous, and can be used to drive design, the user manual, and testing. The Requirements spec is composed primarily of use cases.

A complete description of the behavior of the software to be developed. It includes a set of use cases that describe all of the interactions that the users will have with the software. In addition to use cases, the SRS contains functional requirements, which define the internal workings of the software: that is, the calculations, technical details, data manipulation and processing, and other specific functionality that shows how the use cases are to be satisfied. It also contains nonfunctional requirements, which impose constraints on the design or implementation (such as performance requirements, quality standards or design constraints).