UML System Sequence Diagram
The UML system sequence diagram (SSD) illustrates events sequentially input from an external source to the system. The SSD will define the system events and operations. System sequence diagrams are a timeline drawing of an expanded use case. Events are related by time with the top events occuring first. System events are the important items. These are events that cause a system response.
Use case text may be placed on the left side of the system sequence diagram if desired. If this is done it is best if the use case information lines up with the events in the system sequence diagram.
There may be more than one actor to the system. An actor may be an external automated system that the system may communicate with. Automated actors or robots are shown as actors with a line horizontally through the head.