The initiating actor of a use case is the one that sets the use case in action. All other actors that subsequently take part in the use case are called participating actors. To show this distinction in use-case diagrams, I draw an arrow from the initiating actor to the use case symbol. I then draw simple lines from the use case symbol to all its participating actors.
We also call the primary actor of the use case to the one to whom the use case provides sufficient value to justify its construction. Other actors that collaborate to the successful operation of the use case are called secondary actors of this use case. You could choose to show this distinction in use-case diagrams by using a note on the actor. I usually don't.
Frequently the primary actor of a use case is also its initiating actor, but that doesn't have to be so. Take for example a batch operating use case that generates a management report...
- primary and participating actor: manager (interested in the report)
- initiating and secondary actor: batch starter (probably a scheduled job or an operator)
