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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.
Requirements are what the system must do. They can be lower level, detailed requirements, and they can be functional or non functional. Specifications are those things the system as-built actually does. E.g. you could have a requirement that states the system shall have behaviour X at â10Â°C.