Specifically, a System Sequence Diagram ( SSD ) is a sequence diagram often used to show the interactions between the external actors and the system under design ( SuD ), usually for visualizing a particular scenario of a use case.
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A sequence diagram shows, as parallel vertical lines ( " lifelines " ), different processes or objects that live simultaneously, and, as horizontal arrows, the messages exchanged between them, in the order in which they occur.
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A message sent from outside the diagram can be represented by a message originating from a filled-in circle ( " found message " in UML ) or from a border of the sequence diagram ( " gate " in UML ).
