Maybe you should re write Ardu pilot documentation. Some concepts are poorly explained.
So, I would re write the relationship this way;
Definition: Any channel on the Transmitter (Tx) can act as relay "trigger " function “or” for any other available features inside PixHawk FC. To accomplish this, it requires few steps to build the relationship between the Tx and Pixhawk output Aux Port pins (Actual Pins).
Step 1: Inside the Tx assign e.g. Channel $ ( where $ is 1 to available number of channels) to Logical Auxiliary Output port n (1 to 6) which you want to behave as a Relay. e.g. Tx CHAN 7 = AUXOUT2. This doesn’t mean at this point that Aux Pin 2 will act as Relay output inside the PixHawk. All we have done is we told the Tx to send PWM relay signal to PixHawk FC using AUXOUT2 channel.
Step 2: Next, set RC$_Option =34 via mission planner, where $ is the channel number selected inside the Tx (e.g. 7) and 34 tells mission planner that signal coming from the Tx CHAN 7 should be used as relay signal. Now PixHawk knows that any signal coming from Chan 7 should be considered as Relay signal but next we need to tell PixHawk which of the Aux output Pins should act as Relay pin.
Step 3: The Aux port assigned inside the Tx and which Actual out put Aux pin on the Pixhawk act as relay is done via one single parameter change. To accomplish that, use;
RELAY_PINz = Value
Where z = The actual Aux Pin (1 to 6) in the Pixhawk.
Value = Is what defines which Aux port has been used on the Tx to send the signal.
Value = 50 =>AUXOUT1 = RC9 inside your radio.
Value = 51 =>AUXOUT2 = RC10 inside your radio.
Value = 52 =>AUXOUT3 = RC11 inside your radio.
Value = 53 =>AUXOUT4 = RC12 inside your radio.
Value = 54 =>AUXOUT5 = RC13 inside your radio.
Value = 55 =>AUXOUT6 = RC14 inside your radio.
I hope I got it right.