Editing Software documentation (section)

=== Requirements documentation ===
[[Requirement]]s documentation is the description of what a particular software does or should do. It is used throughout [[Software development|development]] to communicate how the software functions or how it is intended to operate. It is also used as an agreement or as the foundation for agreement on what the software will do. Requirements are produced and consumed by everyone involved in the production of software, including: [[end user]]s, [[customer]]s, [[project manager]]s, [[sales]], [[marketing]], [[software architect]]s, [[usability engineering|usability engineers]], [[interaction design]]ers, [[software developer|developer]]s, and [[Software testing|testers]].

Requirements come in a variety of styles, notations and formality. Requirements can be goal-like (e.g., ''distributed work environment''), close to design (e.g., ''builds can be started by right-clicking a configuration file and selecting the 'build' function''), and anything in between. They can be specified as statements in [[natural language]], as drawn figures, as detailed [[mathematical formula]]s, or as a combination of them all.

The variation and complexity of requirement documentation make it a proven challenge. Requirements may be implicit and hard to uncover. It is difficult to know exactly how much and what kind of documentation is needed and how much can be left to the architecture and design documentation, and it is difficult to know how to document requirements considering the variety of people who shall read and use the documentation. Thus, requirements documentation is often incomplete (or non-existent). Without proper requirements documentation, software changes become more difficult — and therefore more error prone (decreased [[software quality]]) and time-consuming (expensive).

The need for requirements documentation is typically related to the complexity of the product, the impact of the product, and the [[Service life|life expectancy]] of the software. If the software is very complex or developed by many people (e.g., mobile phone software), requirements can help better communicate what to achieve. If the software is safety-critical and can have a negative impact on human life (e.g., nuclear power systems, medical equipment, mechanical equipment), more formal requirements documentation is often required. If the software is expected to live for only a month or two (e.g., very small mobile phone applications developed specifically for a certain campaign) very little requirements documentation may be needed. If the software is a first release that is later built upon, requirements documentation is very helpful when managing the change of the software and verifying that nothing has been broken in the software when it is modified.

Traditionally, requirements are specified in requirements documents (e.g. using word processing applications and spreadsheet applications). To manage the increased complexity and changing nature of requirements documentation (and software documentation in general), database-centric systems and special-purpose [[requirements management]] tools are advocated.

In Agile software development, requirements are often expressed as [[User story|''user stories'']] with accompanying acceptance criteria. User stories are typically part of a feature, or an epic, which is a broader functionality or set of related functionalities that deliver a specific value to the user based on the business requirements.