o start of event actions.
This is the base class for actions that are executed on events.

9.9 Presentation Events
Many objects inside a presentation document support special presentation events. For example, a user can advance the presentation one frame when he clicks on an object with a corresponding event. Presentation events are contained with a graphic object's event listener table. See section 9.2.21 for details.

12.4 Event Listener Tables
Many objects such as controls, images, text boxes, or an entire document support events.
An event binds the occurrence of a particular condition to an action that is executed if the condition arises. For example, if a user places the cursor over a graphic, this condition triggers an action that is supported by the office application.
This event, called "on-mouse-over", can be associated with a macro that is executed whenever the condition occurs, that is, whenever a user places the cursor over a graphic.