What does this statement mean?

PPM receiver is powered from a servo rail, so BEC should be present

That means that all the servo pins on the NAVIO+ board is powered from an external source. Most speed controllers give power through the servo cable to power the receiver and servos, etc. A BEC or Battery Eliminator Circuit is a small power supply that coverts the battery voltage to 5V to power the receiver and servos too.


But what about the servo signal pins?
Will there be a voltage signal here, if there is no power/voltage on the positive servo pins and the only connected power is fthe usb and/or current sensor?


RC output channels will work without a BEC. Only the power pins are affected by it.