A Trigger used in a State Machine is an extension of the basic definition found within the behavioral and structural portions of the specification. A Trigger defines the types of events that can initiate a transition between states. An event is anything that can happen in a system, such as a signal sent by a Behavior, a call to a specific operation, reaching a point in time, or a change in values within the system.

To create the Trigger

• For a State element, in the State dialog box, click Add. The Trigger dialog box opens.
• For a Transition relationship, in the Transition dialog box, select the Trigger box, and click Add (note that the Trigger property is hidden by default). The Trigger dialog box opens.

Specify the Trigger element in the Trigger Specification window.  
 
In the Trigger dialog box, near the Event box, click the ... button and select existing or create new Event.