A use case is a requirements analysis technique that describes how an external actor interacts with a software system to achieve a specific goal. In software quality engineering, use cases help clarify functional requirements by showing user-visible behavior, normal flows, alternate flows, exceptions, inputs, and expected outcomes. They are not intended to show the internal view of the system, because implementation details are usually hidden from users. They also do not represent the software design itself, although designers may later use them to create design elements. The steps taken by the developer are part of development procedure, not use-case modeling. Since use cases illustrate what the system does from the user or actor perspective, they describe system functionality.

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