FileNet® P8 optionally runs user-defined code in response to system or custom events. This user-defined code is called an event action handler, which you register with FileNet P8 as an event action. By using a subscription, you associate an event action with one or more events and objects.
initiating action => event fired on the source object => event action (and workflow in some cases)
For example, a developer might code an event action handler that sends an email to system administrators to inform them about the deletion of an object. After registering this code as an event action, you associate the event action with the DeletionEvent event for the Document class by creating a subscription. Deleting a document is the initiating action that causes the DeletionEvent event to fire, which in turn causes the event action to run.