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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.Functional requirements are usually in the form of "system shall do <requirement>", an individual action or part of the system, perhaps explicitly in the sense of a mathematical function, a black box description input, output, process and control ...
Within systems engineering, quality attributes are realized non-functional requirements used to evaluate the performance of a system. These are sometimes named architecture characteristics, or "ilities" after the suffix many of the words share. They are usually architecturally significant requirements that require architects' attention. [1]
ISO/IEC 9126 distinguishes between a defect and a nonconformity, a defect being "The nonfulfilment of intended usage requirements", whereas a nonconformity is "The nonfulfilment of specified requirements". A similar distinction is made between validation and verification, known as V&V in the testing trade.
Software's functional quality reflects how well it complies with or conforms to a given design, based on functional requirements or specifications. [1] That attribute can also be described as the fitness for the purpose of a piece of software or how it compares to competitors in the marketplace as a worthwhile product. [2]
The type of requirements that relate to performance engineering are the non-functional requirements, or NFR. While a functional requirement relates to which business operations are to be performed, a performance related non-functional requirement will relate to how fast that business operation performs under defined circumstances.
FURPS is an acronym representing a model for classifying software quality attributes (functional and non-functional requirements): Functionality - Capability (Size & Generality of Feature Set), Reusability (Compatibility, Interoperability, Portability), Security (Safety & Exploitability)
ISVV goes beyond "traditional" verification and validation techniques, applied by development teams. While the latter aims to ensure that the software performs well against the nominal requirements, ISVV is focused on non-functional requirements such as robustness and reliability, and on conditions that can lead the software to fail.
Within the application boundary, non-functional requirements are associated with relevant categories and their sub-categories. Using a standardized set of basic criteria, each of the sub-categories is then sized according to its type and complexity; the size of such a requirement is the sum of the sizes of its sub-categories.