Functionality describes the capabilities and behaviours that a system provides.
It includes actions, calculations, responses, and interactions. Therefore, it explains what users and external systems can achieve.
Functional requirements define these capabilities in detail. For example, an online shop may allow customers to search for products, place orders, and make payments. Acceptance criteria then define the expected results.
A function usually describes one specific capability. In contrast, a feature may combine several related functions. Moreover, use cases can show how actors use them to reach a goal.
Functionality differs from quality. It defines what the system does. Meanwhile, quality requirements define how well the system performs.
Clear functionality supports requirements analysis, solution design, implementation, testing, and acceptance.
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