“Steak” isn’t a grade, it’s a cut. It doesn’t come on cans, but is found at the seafood counter. Salmon, for example, is often sold in three forms: whole fish, steaks (transverse cuts of the body), sides (longitudinal cuts [from head toward the tail] on one side of the skeleton, including the skin) and true filets (without the skin) at the seafood counter of most well-stocked markets. True filets are essentially boneless. Steaks and sides hold up better on a grill. Sides and other longitudinal cuts of some fish (e.g. shad) are not boneless, depending on their skeletal structure.
"Fresh"chunks are slightly suspect in many areas, because it is believed that they are used to either salvage or quickly sell fish that are slightly past their peak. Now that fresh or “previously frozen” seafood are widely available even a thousand miles inland, I suspect that it’s simply marketing: It’s awkward to offer a specific type/cut of fish that was swimming free yesterday or the day before, alongside a [cheaper] offering that’s still be okay, but is a few days older. [Though fishhandling and consumer knowledge have improved in recent years, markets are still known to behead cut up whole fish whose eyes betray that they have passed the very pinnacle of freshness.) Most people have heard that fresh fish should never smell or taste fishy, yet I’ve seen many a fish counter, mere miles form the ocean here in Boston, that was fishier than plumber at Watergate!)
Tuna in a can is cooked, not raw like fish market steaks/filet. If you get a chance to savor the almost illegally butteriness of good Buri (yellowfin tuna sushi) you’ll find it has no relation to canned tuna. Not only is raw tuna very different from canned, but sushi is the highest grade of tuna: the slightest defect would be immediately evident in a raw and unadorned piece of fish).
I’m not a snob. A good canned tuna is wonderful in salad, casserole or sandwich. It’s just that fresh tuna is a completely different ingredient; you’d hardly know they were the same creature if you weren’t told. Canned tuna also comes in solid [essentially one complete piece from a steak) to chunk (larger intact pices and some flakes) to flake (very small pieces), but those are sorting grades, more than an indication of quality. ‘Better’ fish do tend to be used for solid vs. flake, simply because opening a can that contains a large blemish could put you off the brand.
BTW, though there can be a wide range of quality in “previously frozen” fish, ship-frozen fish can be quite good, especially since few of us buy our fresh fish when the boat comes in). In fact, all sushi-grade sea fish in Japan is blast-frozen to -60 degrees on the ship (It’s a legal requirement, because it kills fish parasites). I suspect that the American mania for “fresh, never frozen” sushi on the 80’s was one reason why we had more health problems with it than the Japanese did, though they easily ate 100 times as much [net, and per capita] as we did.