New PNP model leads 1,2,3,4,6...what's #6?

[Wingnutz]

Hooked the leads to the Rx in my new PNP model as follows;
#1 to Aileron, #2 to elevator, #3 to throttle, #4 to rudder and left #6 unplugged as there are no flaps or retracts.
Model would not bind, ESC beeped twice, paused and started beeping continuously, motor would not start.
In desperation, I plugged lead #6 into the gear port on the Rx...and everything worked! I have no idea why. Any explanations?
Puzzled