It is confusing for sure. For DCS it only supports two VR APIs, (1) the Oculus SDK or (2) SteamVR (aka OpenVR).
If you have a Windows Mixed Reality (WMR) headset like an Odyssey or a Reverb then for DCS VR you have to use the SteamVR access (via something called WMR for SteamVR, which essentially ‘fakes’ your headset to appear as a SteamVR device).
Just because it’s SteamVR (or the proper name of OpenVR) then it doesn’t mean you need to run the game on Steam. DCS standalone can run OpenVR just fine.
The new kid on the block is ‘OpenXR’. This is meant to be a unifying API that gets rid of all this ‘Is it Oculus, is it SteamVR, is it WMR?’. It’s new and the only thing that supports it so far is MS Flight Sim. There’s a diagram of how it will work around here:
In the future it would be great if DCS, IL-2 Box, XP11 etc add support for OpenXR.