What are System Sequence Diagrams - UML

Use cases describe how external actors interact with the software system we are interested in creating. During this interaction an actor generates system events to a system*, usually requesting some system operation to handle the event. For example, when a cashier enters an item's ID, the cashier is requesting the POS system to record that item's sale (the enterltem event). That event initiates an operation upon the system.

The use case text implies the enterltem event, and the SSD makes it concrete and explicit. The UML includes sequence diagrams as a notation that can illustrate actor interactions and the operations initiated by them.

A system sequence diagram is a picture that shows, for one particular scenario of a use case, the events that external actors generate, their order, and inter - system events.All systems are treated as a black box: the emphasis of the diagram is events that cross the system boundary from actors to systems.

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