Requirements Elicitation
Functional requirements define the internal workings of the software: that is, the calculations, technical details, data manipulation and processing, and other specific functionality that show how the use cases are to be satisfied. They are supported by non-functional requirements, which impose constraints on the design or implementation (such as performance requirements, quality standards, or design constraints).
As defined in requirements engineering, functional requirements specify specific behaviors of a system. This should be contrasted with non-functional requirements which specify overall characteristics such as cost and reliability. (An alternative view is that functional requirements specify specific behavior while nonfunctionals provide adjectives which may be used to describe these behaviors.)
Typically, a requirements analyst generates functional requirements after building use cases. However this may have exceptions since software development is an iterative process and sometimes certain requirements are conceived prior to the definition of the use cases. Both artifacts (use cases documents and requirements documents) complement each other in a bidirectional process.
A typical functional requirement will contain a unique name and number, a brief summary, and a rationale. This information is used to help the reader understand why the requirement is needed, and to track the requirement through the development of the system.
The core of the requirement is the description of the required behavior, which must be a clear and readable description of the required behavior. This behavior may come from organizational or business rules, or it may be discovered through elicitation sessions with users, stakeholders, and other experts within the organization. Many requirements will be uncovered during the use case development. When this happens, the requirements analyst should create a placeholder requirement with a name and summary, and research the details later, to be filled in when they are better known.
Software requirements must be clear, correct, unambiguous, specific, and verifiable.