Do you mean "bind plug"? It's used to tell a spread spectrum receiver that you want to "bind" it with a transmitter. With the Spektrum DX6, you put the bind-plug on the battery connector on the RX, turn on the receiver, then turn on the transmitter with the bind button pressed (back of the transmitter). This "binds" the receiver to the transmitter (the receiver will only respond to the transmitter that it was "bound" to based on a unique key code in in the transmitters signal).

Steve