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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.
The Functional Requirements Specification documents the operations and activities that a system must be able to perform. Functional Requirements should include: 1  Descriptions of data to be entered into the system. 2  Descriptions of operations performed by each screen.3  Descriptions of work-flows performed by the system. 4  Descriptions of system reports or other outputs.f a User Requirement Specification was written, all requirements outlined in the User Requirement Specification should be addressed in the Functional Requirements Specification. The Functional Requirements Specification should be signed by the System Owner and Quality Assurance.