The difference between fatty tuna is what part of the tuna you are taking it from. Fatty tuna is coming from the belly part. It's more oily. The color usually becomes a pinkish color, and the backside of the meat is reddish because there's not much fat in it. It's more like muscle.
It used to be that even in Japan, we only ate the back part of it because the fatty part of it gets bad so quickly. We didn't always have any refrigeration system, and it would just get bad. People would throw it away or bring it to their cats.
The homeless people and whoever didn't have much money, started eating those things, because it was being thrown away. They said it was very good. This is how it became more popular and popular in my country.
This country likes the fatty part of the tuna. People were throwing it away because they didn't know what to do. Now, it's considered the best part of the tuna, and the most expensive part of the tuna.
When you are mixing with a spicy miso sauce, or a heavy kind of sauce, don't even try to use the good part of the tuna, because you will only taste the spicy sauce. Get the red tuna, the inexpensive tuna, and mix in with the sauce.
If you get the real good part of the blue fin or a big eye tuna, or the medium belly part, don't mix it with anything. Just try to eat it as it is. Try to taste the good flavor over the flesh. That's the best.