As answered by others the choosing of BT or DAC priority can only be set for the BTR5 in the apps, and the default priority is BT first - if there is a paired BT device the BTR5 will auto-reconnect to the BT device, if not the BTR5 stays in "Reconnect" until you connect a USB DAC device, then the BTR5 will connect to it.
I found the default of BT Priority useless for me as I wanted to be able to automatically switch between USB DAC audio and BT audio, and once connected to a BT device the BTR5 will ignore the USB DAC audio stream and only use the USB connection for charging (if you have the BTR5 Charge = On). BT Priority mode was less useful to me if I had to go to all of my BT devices and disable BT before starting to use USB DAC audio.
If a BT device comes into "view" the BTR5 will auto-reconnect to that BT device even if you are in DAC Mode, which made the BT priority mode behavior less useful for my own personal use. But, if you want to switch over to your phone's BT for a call you could enable BT on your phone when you want to make or take a call - which would then cause the BTR5 to automatically connect to the phone while listening on the USB DAC - leaving BT off on all your devices otherwise to allow the BTR5 to connect via USB DAC.
If the BTR5 is set to DAC Priority, when you are walking around listening to your Phone BT or PC BT - and you sit down and connect your BTR5 to a USB DAC cable on your PC or USB DAC on your phone, the BTR5 will switch to processing audio on USB and "silence" but not disconnect from your BT device - this can be confusing - as you won't actually be able to use the BT device while connected using USB DAC audio - but if you pull out the USB Cable from the BTR5 it will switch to the already connected BT device or "Reconnect" to another available BT device if the BTR5 was not already connected to a BT device.
Yes, it can get confusing. You can get into a state in either Priority Mode where you don't immediately get your desired effect. There are work arounds - or working steps - you can take to get the desired effect easily.
Here are some examples under DAC Priority, the mode I've decided to use, and I do describe BT Priority and why I don't use it as well:
1) When using my PC I leave my PC BT on, that way I can connect to PC USB in DAC mode simply by connecting the PC USB cable - the BTR5 automatically switches to DAC from BT and I can enjoy either DAC or BT just by connecting or disconnecting the USB cable physically - I don't need to do anything else in software, with one exception...
2) I have to make sure that BT is off on any other device than the PC while I am enjoying listening on my PC. Otherwise the BTR5 may use BT mode with another device instead of the PC when I disconnect the USB cable connected to the PC. Which can be confusing, although helpful if you want to switch to your phone when not on PC USB.
3) You may have already figured out this next example, purposely disable BT on your PC so when you disconnect from PC USB DAC your BTR5 will be ready to go mobile connected to your Phone via BT- you will need to disable BT on your PC and only have your Phone BT enabled so that the BTR5 only see's one device available on BT - the reconnect is "random" if you have more than one BT device active.
4) It an get confusing, yes, and it would be "better" if the BTR5 could be connected to more than one BT device at a time along with DAC USB mode priority, such that you could get "calls" while on USB DAC mode without having to disconnect the USB cable from the PC, or if you were to be able to prioritize the BT devices you could default to PC BT while at home (after disconnecting the USB cable) and the BTR5 would switch from PC BT to Phone BT if the phone can interrupt with an incoming call - like I've used on Plantronics devices for years, but the BTR5 doesn't have this dual connected BT order priority feature.
5) What happens if the BTR5 is on USB DAC priority and you are using the Phone USB connection? Same thing. Disconnect the Phone USB cable from the BTR5 and the BTR5 will switch to it's BT connection - likely most usefully to your Phone's BT so you can disconnect the cable from the phone and be mobile with your phone on BT.
6) Why didn't I pick BT Priority? It would require me to turn off BT on my devices (all of them) in order for the USB DAC audio connection to work. And, I found I needed to disconnect the USB cable and reconnect the USB cable again if I forgot to turn off BT on *any* of my BT devices before connecting the USB DAC audio cable, as BT Priority means just that - the BTR5 will ignore the USB data connection if the BTR5 can connect to any BT device it has paired with - the BTR5 might even try to pair with an unknown BT device even if you are already connected on USB DAC if one comes in range... I don't know for sure about this last situation as I haven't tried it, but it just came to mind as a possibility.
The one critical advantage of BT Priority is if you charge your BTR5 via USB cable connected to a device that will support USB DAC and you wish the BTR5 to continue using BT while charging off of that USB DAC mode capable device. As if the BTR5 automatically switched to USB DAC mode while charging only is desired you might miss BT phone calls. This might be where BT Priority is useful, but it's not useful enough for me.
On USB DAC Priority if you want to use your BT Phone for calls on BT you need to physically disconnect the USB cable from your PC or Phone (yup) in order to take or make a call on BT. The good thing is if you do disconnect the USB DAC cable from your PC or Phone to take a call on BT the BTR5 will automatically switch to BT once the USB DAC cable is disconnected.
One more related gotcha - you need to remember if you have Charge=On or Charge=Off when charging the BTR5...
Charge = ON is required to Charge the BTR5 via a USB-C cable connected from a power adapter, I use a 25w Samsung adapter. If you have Charge = Off no USB connection will power or recharge the BTR5. I know that sounds obvious, but what if you are listening to a BT device via your BTR5 and you connect your BTR5 to a power adapter thinking "hey I might as well charge my BTR5 so I can use it longer later"... nope the BTR5 will just ignore that USB power. So if you want to charge your BTR5 with Charge=Off, you must Turn Off your BTR5 before any USB power will work. It's easy to forget what mode your BTR5 is in, so you have to consciously remember to Power Off your BTR5 in order to charge it when Charge=Off.
I know that was long, but I wanted to cover all the gotcha's I've experienced, as it can be confusing or frustrating trying to figure this out on your own the first time through.
If you can't understand something as I've said it please ask right away. Thank you.