They define how objects should respond to an event. For example, a button script may run when a user clicks a button. A page script may run when a user enters page in the book. Scripts are made up of OpenScripts statements which are grouped based on their purpose, usually in response to an event. These groups of statements are called handlers.
Every object can have a script.
A message handler tells ToolBook what actions to perform when the object whose script contains the handler receives a particular message. Message handlers are the most common type of handler and the focus of the workshop. In this example, ToolBook will take the reader to page 5 when the mouse button is released. The handlers within a script can be in any order, because ToolBook calls a handler when an event triggers the message to which the handler responds. The statements with a handler, however, must be in the order in which you want them to be executed.