A lot depends on the species, but basically tails are leaner than loins and should be prepared accordingly. The meat will be firmer (and tougher) than loin.They do not take overcooking at all well. I'd suggest filleting and skinning; then grilling, possibly crusting and serving rare to medium-rare -- ala "seared tuna" (tataki style); or poaching and serving cold, e.g., in salads such as a Nicoise. Perhaps some Asian or Mediterranean soup applications.
Do not "steak," by cutting bone-in cross-sections. Without skin they won't hold together, and tuna skin won't help the flavor any.
Because it's so lean, it's not a highly prized part of a tuna fish. It's often diced and served raw for "spicy tuna," "tuna tartare," and hwe dop bap (I'm freaking you out with that one, aren't I? It's a sort of Korean sushi salad), or western takes on it. You might also consider tuna "kababs" if your restaurant does that sort of things.
If it's a light colored tuna like Albacore, same-same in spades.
Honestly, I'm a little freaked that you don't know what to expect from the tail section of a fish -- as opposed to a loin or collar. Don't you ever butcher medium and medium large wholes, halves or sides? Not your fault, of course. You handle what they put in front of you. Whomever "they" are.
Tantrum over: Grill with as much as heat as possible as fast as possible. Poach with... well helzbellz, you know how to poach. Just watch it. Any kind of overcooking at all, and it's going to taste like catfood. Flaking means waaaaaaaaaaaaaaaay overdone and comes with a "stick to your teeth, taste bad gurantee." Not to be harsh.