The per band (2.4 / 5GHz) has been introduced recently to allow different ARM power settings per band, so you can set the power on 5 GHz slightly higher over 2.4 like advised in the VRDs.
The RF -> ARM is a global setting, so it applies to all radios.
I just tested in my lab, and what happens is that if you have set the RF -> Radio -> Customize ARM Power range, that setting will take precedence. If you have not set it for the specific band, the setting in RF -> ARM -> Min/Max transmit power will be set.
You can also set static channel+power on the AP itself in the Radio tab. That will disable ARM for that radio(s), and configure static settings.