Edit: Realized didn't hit your main question - ORA grades I believe are Ultra / First / Gold [brown] with colors added sometimes e.g. Green First or Green Ultra. "Red sea" is a not a grade but a locality (patterns are geographically distinct, Red Sea maximas have different patterns than Micronesian than Polynesian).
My limited but detectable knowledge is that it's more on the grower side, and so by parent company/wholesaler, but sometimes gets passed through.
For instance, a farm settles X many larvae, lets them attach and grow a bit, and then once mantle colors have started to develop they sort them into different raceways for growout. It's not like the workers there just grab the first 1000 baby clams they see, I believe they have targets (i.e. they expect to ship a certain number of high-priced solid blues, medium priced patterns, etc) so this is how they know who gets grown out. And sometimes this grading designation get passed along and AlgaeBarn will sell you ORA's 1st grade, which is going to be a different parameter than one farmed by PacEast.
Fun fact, not sure if it's still true, but the exceptional ones didn't even get shipped to US, at least when i was aware of things, they went straight to Germany & Japan. I suspect the high-end market has gotten good enough in the US since then that they might come here now, but just an anecdote from years past.