ArduPilot 3.7.1 Flight Modes

With so much obsolete info out there, it is hard to find what’s current. My questions are simple: what channel is used for flight mode selection on 3.7.1 (it used to be ch. 5, seems to be 8 now)? Can it be changed (used to be yes, seems to be no now)? If so, how (used to be a drop down list in configs, NA)?

On Plane this is configurable using the FLTMODE_CH parameter.