Typical interaction scenario from a user's perspective for system requirements studies or testing. In other words, "an actual or realistic example scenario". A use case describes the use of a system from start to finish. Use cases focus attention on aspects of a system useful to people outside of the system itself.

• Users of a program are called users or clients.
• Users of an enterprise are called customers, suppliers, etc.

A collection of possible scenarios between the system under discussion and external actors, characterized by the goal the primary actor has toward the system's declared responsibilities, showing how the primary actor's goal might be delivered or might fail.

Use cases are goals (use cases and goals are used interchangeably) that are made up of scenarios. Scenarios consist of a sequence of steps to achieve the goal, each step in a scenario is a sub (or mini) goal of the use case. As such each sub goal represents either another use case (subordinate use case) or an autonomous action that is at the lowest level desired by our use case decomposition.

This hierarchical relationship is needed to properly model the requirements of a system being developed. A complete use case analysis requires several levels. In addition the level at which the use case is operating at it is important to understand the scope it is addressing. The level and scope are important to assure that the language and granularity of scenario steps remain consistent within the use case.

There are two scopes that use cases are written from: Strategic and System. There are also three levels: Summary, User and Sub-function.