You understood incorrectly, they don’t.
That is the PWM for the flight mode selection switch on your RC transmitter. You use the switch to change the current flight mode.
That is not the RC-out range of the copter motors.
I could find this text on the copter documentation :
If you want to just support three modes (using a three position switch) then you would configure the transmitter to produce PWM pulse widths of 1165, 1425, and 1815 us for the respective switch positions.
can you please explain more why shall anyone set the PWM on transmitter to get these specific PWM values?
These ranges were selected ages ago and are hard coded. You don’t have to use the Flight Mode channel if you can’t understand it’s simplicity. Use RCx_OPTION where “x” is the RC chanell you have mixed on your radio.
You don’t have to do any special setting on the transmitter. Only assign a switch to the channel you want to use.
Than in mission planner you see one of the six fields in green and if you switch on transmitter to another position another field will be green now. By this you can see which positions are available for you and you can set these fields to the modes you want to use
Buy the switch on transmitter side a transmitter specific PWM is send over the mode select channel.
The FC has for each mode position a PWM range and if the received signal is somewhere in this range the assigned mode is selected.