The mtDisconnect event:

The mtDisconnect() handler is called whenever your device realizes that the PC browser has disconnected from your device.

You might use this to perform some sort of cleanup operations: maybe shut down some part of your hardware or maybe set some flags so your code won't try to send any data if it is not connected.

In our simple PICDEM.net2 project, we don't need to add any code to this mtDisconnect() handler.