The value of parameter key is the control key that is about to be sent. In the event handler function you may change the control key value before it is sent to the host. The event can also be canceled by setting the return value of the event handler function to false. The return value false is the default if your event handler does not explicitly set it. If you cancel the event, the control key will not be sent to the host. By setting the function return value to true, the event continues with the control key value in the parameter key, which could be a different control key value set by the event handler.