A Trigger indicates an event that initiates an action (and might arise from completion of a previous action). You initially define a Trigger in one of two ways:
|·||As a property of a Transition relationship|
|·||As an event in a State Machine Table.|
When you save the Trigger, it is added to the list of elements for the parent package in the Project Browser. You can then right-click on it and, if required, edit its properties. You can also drag the Trigger element onto another diagram, although there are limited uses for the element in that context.
This element is not the same as a Trigger Operation, which is an operation automatically executed as a result of the modification of data in a database.
OMG UML Specification
The OMG UML specification (UML Superstructure Specification, v2.1.1, p. 454) states:
Events may cause execution of behavior (e.g., the execution of the effect activity of a transition in a state machine). A trigger specifies the event that may trigger a behavior execution as well as any constraints on the event to filter out events not of interest.