Tuna is typically graded from 1 to 3 with 1 being the best and 3 being the worst. There are also in between grades of 2+ and 3+ and some purveyor's will also use 1+ to denote the "best of the best". When people talk about "sushi grade" or "sashimi grade" tuna, what they're referring to is a #1 grade of tuna. To be eaten raw or rare, I wouldn't go any lower than a #2+ for health reasons but you really do want a #1 grade for aesthetics, flavor and texture. Also, grading tuna is highly subjective and it's important to buy tuna from someone who has the same level of standards or grading perspective that you do. Some purveyors or fish markets will try to pass off a #2+ as a #1 or a #2 as a #2+.
Also, just as equally important when buying tuna is knowing what type of tuna it is. The three most popular tunas are bluefin, bigeye and yellowfin. They differ in color, taste and fat content as well as price.
The most common type is yellowfin, or otherwise widely known to the public as "ahi tuna" and is the most widely used in restaurants. It's also the leanest, the cheapest and least flavorful of the three listed above.
The next fattiest tuna type is the bigeye and is a big step up from yellowfin, with a richer flavor profile and much redder color.
The fattiest of them all, and most expensive, is the bluefin tuna.
The color difference between bigeye and bluefin isn't as straightforward as is the case with yellowfin. Sometimes bigeye can appear to be a deeper ruby red versus the purer red of the bluefin. Generally speaking, visually, I like the look of bigeye but flavorwise, prefer bluefin.
When I make any type of sushi, sashimi, tartare or any type of raw tuna dish, I use only bigeye or bluefin and always use no less than a #1. If you're cooking tuna to medium rare or more, a 2+ is acceptable.
Thanks!
Another question, sorry.
What "level" of tuna do you need to buy to do something like that cucumber tartare?