About RFD900X "set ppm fail safe "

This is the PPM stream that is recorded to be played when the aircraft goes out of range.

If you are using PPM signalling through the radio for RC control it records the stream you would like played in the event of link loss.

Generally this would be a throttle out of range to let the flight controller know there is a problem and invoke some action like return to launch.

HHHHHH china no.1