About, what are the numbers on the right of the sprite name? I don't mean the sprite number, the numbers on the right of the sprite name. You know for example, Mega Goomba has 16-3.

The sprite set group number, and the number inside the group.

What exactly is a group?

It has to do with the sprite sets.

16 means that is the 16 option of the sprite set list that will make the sprite work.

3 means that is the third option in the 16 box of sprite sets.
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Look at the sprite set configuration tab.
It has 11 select boxes, numbered 1-10 and 16. Each one is a "group".
You can select ONE sprite set from each group. Each one has a number. That'd be the "sprite set number" inside the group.