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Broadly, functional requirements define what a system is supposed to do and non-functional requirements define how a system is supposed to be mathematical function, a black box description input, output, process and control functional model or IPO Model.roadly, functional requirements define what a system is supposed to do and non-functional requirements define how a system is supposed to be mathematical function, a black box description input, output, process and control functional model or IPO Model.
Functional specification.: The functional specification (usually functional spec or just spec for short) is the formal response to the objectives. It describes all external user and programming interfaces that the product must support.