I presume you are working on a fixed wing configuration of PX4?
If so, it will be the radio channel connected to the flaps, if any, on your vehicle. YOu can find out about flaps here: https://en.wikipedia.org/wiki/Flap_(aeronautics)
These control the lift at a particular speed. They aren't tied to yaw, roll, thrust, so you need a separate physical control on your transmitter for them.