i think its a complete marketing ploy. like using "rainbow" on a coral with 3 colors in it I don't know what rainbow they are looking at but it sounds pretty sad.
Its all in the eye of the beholder. I see Ultra grade is an opinion. What you see as ultra I may not. As for rainbow, coral has been classified as rainbow if it has 3 or more colors.
I think "ultra" was first used on clams... any clam that is blue is considered "ultra" now! Then when aussie corals first started coming in, ultra was applied to them. But in general, if you see 100 orange plates, and 15 or so are much more colorful than the rest, then you can consider those "ultra". Really its a term misused in wholesale as much as retail.