Well, Sir Herbert, I agree, you just used different words!
I can remember arming RNAV approach well above 10000 feet with no adverse consequence
- that is no problem, as I explained above, because the FMS will just fly whatever you programmed into it.
You guys should be flying something that has the pilot make the choice of "NavSource", like my little CRJ. That makes it obvious what kind of signal is being used to compute autopilot commands.
Nic