One of the steps that you take to use a 2.4 Tx &Rx (speaking from Spektrum DX7 technology) is to "bind"the Tx with the Rx. The Rx is powered up with a bind plug inserted. The Rx's software (firmware actually)has code that recognizes that the bind plug has been inserted. It is now in a receptive state to receive a bind command from a Spektrum-enabled Tx providing the Rx with the Tx's unique identifier and model number. The Tx is then turned on while simultaneously holding the "bind" button on the back of the Tx. The Tx's software/firmware recognizes that the bind button is being pushed and transmits its unique id and model number. The Rx receives the bind message and stores the Tx's unique id and model number in non-volatile memory. I'm guessing that there may be a couple of back-n-forth communications between the Tx and Rx during the bind process to attempt to ensure an accurate connection, but that's just a guess.

Ican bind my Tx to 20 different Rx's (20 different models). Ican give that same Spektrum Rx to someone that has a transmitter that will work with a Spektrum Rx. He can then bind that Rx to his Tx / model. After he binds HISTx to the Rx my Tx can no longer communicate with that Rx (unless, of course, I rebind the Rx to MYTx).

HTH - clear as mud, huh