The identifier is a unique number created when the RegisterEventHandler function is used. This identifier is used by the UnRegisterEventHandler function to identify which handler you want to unregister. For example:

var myEvent = image1.RegisterEventHandler("lclick", this.Move)

The variable myEvent now contains the identifier (a number e.g. 1) that can be used to unregister this new event handler, like so

image1.UnRegisterEventHandler(myEvent)

The handler is unregistered because the variable myEvent contains the identifier required to unregister this handler.