From
http://www.wsj.com/articles/amd-pric...ket-1464725394 (stick the link into google and open it from there - they don't paywall hits through google):
"AMD said its new Radeon RX cards, certified for use in VR by HTC and Oculus VR, deliver performance equivalent to that of $500 graphics cards used for VR."
I took that to mean RX cards are certified by HTC and Oculus. Cards that are either not powerful enough or not certified for some other reason could still be R3/7/9. Or perhaps I'm reading too much into it.