Tuna is unofficially graded in a very neanderthal way, the fish house auctioneer cuts a chunk out of the tail and grade it based on the color of the meat. It really doesnt get much less official than that. If the auctioneer/grader needs a high grade to sell and drive the prices up or level out the market in the auction that that is what they do.
If the tuna is a nice deep ruby red and it is solid and fresh than go for it.
Taste: The sensation derived from food, as interpreted thru the tongue to brain sensory system. Flavor: The overall impression combining taste, odor, mouthfeel and trigeminal perception.