9.2. System Sequence Diagrams
Use cases describe how external actors interact with the software system we are interested in creating. During this interaction an actor generates events to a system, usually requesting some operation in response. For example, when a cashier enters an item's ID, the cashier is requesting the POS system to record that item's sale. That request event initiates an operation upon the system.
It is desirable to isolate and illustrate the operations that an external actor requests of a system, because they are an important part of understanding system behavior. The UML includes sequence diagrams as a notation that can illustrate actor interactions and the operations initiated by them.
A system sequence diagram (SSD) is ...
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