The scheme used by the transmitter to encode its signal. The protocol is proprietary and is what determines whether or not your radio is a Futaba, a JR, an Airtronics, a FrSky, or a Joe Blow Special. If your transmitter has an interchangeable module in back (the Taranis does!), you may be able to purchase an add-on module that allows you to use a different protocol. The Taranis accepts any proprietary module that meet the JR interface specification.