[1] Functional requirements may involve calculations, technical details, data manipulation and processing, and other specific functionality that define what a system is supposed to accomplish.
This should be contrasted with non-functional requirements, which specify overall characteristics such as cost and reliability.
The hierarchy of functional requirements collection and change, broadly speaking, is: user/stakeholder request → analyze → use case → incorporate.
A typical functional requirement will contain a unique name and number, a brief summary, and a rationale.
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.