Functional requirements may be calculations, technical details, data manipulation and processing and other specific functionality that show how a use case is to be fulfilled. They are supported by non-functional requirements, which impose constraints on the design or implementation (such as performance requirements, security, or reliability).
As defined in requirements engineering, functional requirements specify particular behaviors of a system. This should be contrasted with non-functional requirements which specify overall characteristics such as cost and reliability.
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.
The crux of the requirement is the description of the required behavior, which must be clear and readable. The described 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 may 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.