A set of uses relationships in the model. A sequence of transitions that are performed by the system. A state machine for the complete system. An Actor must be directly associated with at least one use case in the system. A use case can be related to actors and other use cases.
Actors can be people, machines, or other systems.
It represents one flow of events in the system. A software designer wants to show a scenario that has a time critical flow of control between multiple objects. When a state has one non-qualified outgoing transition. When a transition does not go to a state. When a transition does not have a guard condition.