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Functional specification
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<p>A functional specification (also, <i>functional spec</i>, <i>specs</i>, <i>functional specifications document (FSD)</i>, <i>functional requirements specification</i>) in <a href="/facts/Systems_engineering/gFwRmkDp">systems engineering</a> and <a href="/facts/Software_development/UUfsAaYY">software development</a> is a document that specifies the functions that a system or component must perform (often part of a requirements specification) (ISO/IEC/IEEE 24765-2010). </p><p>The documentation typically describes what is needed by the system user as well as requested properties of inputs and outputs (e.g. of the <a href="/facts/Software/RP79lVSW">software</a> system). A functional specification is the more technical response to a matching requirements document, e.g. the <a href="/facts/Product_requirements_document/0cvY5g16">product requirements document</a> "PRD". Thus it picks up the results of the <a href="/facts/Requirements_analysis/fHtWUoMs">requirements analysis</a> stage. On more complex systems multiple levels of functional specifications will typically nest to each other, e.g. on the system level, on the module level and on the level of technical details. </p>