Why in the world has anyone not produced a simple table that shows what's allowed and what isn't? Our illustrious aviation regulators simply do not want the responsibility. They'd rather keep things as is so they are open to interpretation. That keeps the airlines happy which is all they care about really.
I've just had an awkward sim session with a seasoned trainer completely at odds with everything I've been taught about RVRs at my previous gig and not a single legal document exists to clarify the disparity. What a stupid stupid position we find ourselves in in the year 2018.