5 Use cases are discovered by identifying productive task results. Subtasks leading to those results become events within an identified use case. Each approach offers advantages. Shall statements allow formal integration of nonfunctional and architectural concerns into analysis and provide specific reference to requirements, e.g., performance response times, that may apply to multiple use cases. Scenario stories offer a completeness of detail and an effective user validation viewpoint that is difficult to achieve with other approaches.
Scenarios are also of great value in obtaining eventual user acceptance of new or changing systems.
Finally, no matter which use cases are identified, they cannot be put together until the actors and collaborators in events are identified. Each approach also has limitations. It is sometimes difficult to obtain mission focus from untrained subject matter experts, even in facilitated workshops. Merely getting consensus on the mission can be an interesting task in some environments. Theoretically, shall statements are individually verifiable and can be clearly written, at least in the microsense.