Arrows entering from the left edge of the diagram are external triggers that originate outside of this decomposition. An arrow exiting the node is an output that triggers another function. An arrow entering the node triggers that function. The arrows on a sequence diagram represent control data, triggers that serve to synchronize interactions. Individual function nodes are then placed along the corresponding vertical lifeline with unallocated functions displayed on a special 'unallocated' lifeline on the right side of the diagram.

The allocation of these functions (the allocated to relationship) determines the collection of components displayed along the top of the diagram. The individual child functions are identified by traversing the parent function's structure (best represented graphically by an activity diagram or an EFFBD). The sequence diagram displays the children in a function's decomposition along vertical lifelines. The sequence diagram is available for entities in the Function class (as well as any other subclasses of ProcessingUnit). This partial representation is particularly useful in understanding and confirming interactions early at each level of the system definition effort. Part of the behavioral (logical architecture) representation set, the sequence diagram enables you to focus on triggering data and the resultant flow of control between components. The sequence diagram captures the interaction between collaborating parts of a system. Sequence Diagram (available in the SysML base view set)

Sequence diagram online