I’ve contacted Leeuwarden airbase in the Netherlands. One of their active MLU pilots confirmed that the UHF function knob only has meaning when CNI is in backup mode. As he stated, the function knob turns on the UHF panel when set to MAIN or BOTH. (In backup mode, COM1 is still used to set volume). So, although you can still turn the UHF panel on and off via that knob, it doesn’t effect UHF communication when CNI is set to UFC. That means that the current implementation in BMS is incorrect, at least for the MLU. Could as well be different for other blocks.
So, how I think it should be implemented: Setting CNI to UFC and turning COM1 on should be enough for UHF communication. When setting CNI to backup, you must use COM1 (for volume) and the UHF function knob (other that OFF) for UHF communication. When the UHF function knob is set to OFF, the frequency display at the UHF panel is off. The video (at 00:31) in the fourth post of this topic shows that.
Of course it’s up to you what to do with this information. For me this subject is finished after this post, as I’m not looking for a yes-no-debate. The function knob is only a single click during ramp start, so it’s not really a big deal for me. Just wanted to contribute to this awesome game. Thanks for developing it!!