Use case scenarios represent the interactions of the system executing the use case with the actors. Each scenario
represents a specific sequence of events, messages, and actions. Each of these elements captures at least part of a
requirement, and often more than one requirement, specifying a required or optional sequence, input and output events
and messages, and control and data transformations.
A use case normally has between several and several dozen such scenarios and each scenario has typically several to
several dozen messages. The scenarios set is said to be 'minimally complete' when each functional or quality of service
requirement is represented on at least one scenario. Messages between the use case and the actor(s) are normally
represented as asynchronous events that optionally carry data. This constitutes the so-called 'logical interface'
between the actors and the system as it executes the use case.
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