Activity diagrams are used to specify use-cases and in
operations implementations. They show the sequence of
activities in algorithm implementations. Generic diagrams can have
branching controlled by guards. Completion of one
activity may automatically (trigger-less) lead to another
activity. Send clauses may send action messages to trigger
other activities. UML 1.3 no longer uses ^ for send clauses.
Start pseudo-activity may send an action message. Similarly an
action message may take to the stop pseudo-activity.