Events are notifications, or messages, that are sent out by AutoCAD to inform you about the current state of the session, or alert you that something has happened. For example, when a drawing is opened the BeginOpen event is triggered. This event contains the name of the AutoCAD drawing that was opened. There is another event triggered when a drawing is closed. Given this information you could write a subroutine, or event handler, that uses these events to track the amount of time a user spends working on a particular drawing.