RMV: Respiratory Minute Volume. This is the volume of gas that a given diver actually moves in and out of their lungs in a minute at any point in the dive. It is dependent on the diver and on depth.
No, it is not dependent on depth.
Nothing in "RMV" per se requires it to be at the surface.... RMV is ambiguous
Right. Nothing requires it to be at the surface - because it doesn't matter what depth you are at. Therefore, it is completely unambiguous.
If I breathe in and out 6 times in 1 minute and that results in me cycling 0.5 cu-ft of air through my lungs, then I am going to cycle 0.5 cu-ft of air whether I am sitting at my desk or whether I am hovering above a reef at 33' depth. Either way, my RMV is 0.5 cu-ft/min. Depth doesn't matter and the measurement is completely unambiguous.
The only thing that depth matters for is if you measure the RMV at one pressure and you want to figure out what volume of gas that would translate to at a different pressure.
For example, if you cycle through 0.5 cu-ft of air in 1 minute while you are at 33' depth and you want to know how much gas you have left in your tank. If it was a full 100 cu-ft tank, that means the gas inside would occupy a volume of 100 cu-ft if allowed to expand until it was at 1 ATA. At 33', the ambient pressure is 2 ATA, so the gas in your tank would only expand to 50 cu-ft of volume. You used 0.5 cu-ft, so you just used 1%. You have 49.5 cu-ft left at your current depth. If you ascend to the surface, you would have 99 cu-ft left.
Depth determines the gas pressure. The pressure, the gas composition, and the volume determine the mass. So, the mass of the gas you breathe is dependent on the depth. The volume is not dependent on depth. Thus, Respiratory Minute Volume does not depend on depth - only time and volume (lung tidal volume and breathing rate) - and is unambiguous.
OTOH, SAC is very ambiguous because the number of psi per minute is complete meaningless without also stating the cylinder water volume and working pressure - or the cylinder's surface air volume - you are using.
Anyway, IIRC, my TDI manuals defined RMV and SAC the way I use them.
Also, if there are two common terms used, where one is for volume per minute and the other is pressure per minute, and one of the terms is unambiguous that it refers to volume per minute (Respiratory
Minute Volume), and the other is ambiguous regarding whether it is a measurement of pressure or volume, then it seems to me to make the most sense to use the unambiguous term for what it says it is and take the other term to mean the other thing. But, that's just me and obviously not everyone agrees. I do wish I could find a sensible explanation of a reason to do it some other way, though (where, "because that's how I was taught," would not be considered a sensible explanation).