The exclamation mark (!) means that there has been no communication with the device for the past two minutes. The driver is logged in, but the device is currently not providing GPS data to the Driver App.
The device is providing GPS data to the app, but it is not stable and/or is of very poor quality.
The asterisk sign (*) means that the the device is providing GPS data to the app, but it is not stable and/or is of very poor quality