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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.
1 Functional specification. 2  The functional specification (usually functional spec or just spec for short) is the formal response to the objectives. 3  It describes all external user and programming interfaces that the product must support.