Objects communicate as a result of state machine instances sending signals. To synchronize the behavior of objects, actions in state machine instances generate signals to establish the dynamics of a domain.
Definition: A signal is a message that may carry data used by the actions in a procedure of the recipient's state machine instance.
The signaling is asynchronous. Once the signal is sent, the sender goes on about its business. Separately, once the event is detected by the receiver, a transition is made and the receiver executes a procedure. The receiver does not “return” to the sender, though it may choose to send another signal.
Signal actions send signals to objects. In Figure 10.1, a signal is sent ...