They can be what you want them to be with a few exceptions.
0-15 all work so that they transition into each other.
16 and 17 only work with texture 2.
18-20 are coast lines and they only transition with 1.
Here is an example of how the textures for SFMA are laid out:
0 water
1 grass
2 grassy rock
3 farm
4 rock
5 forest
6-15 are all transitions
Here's a chart of how the textures transition into one another.
The top row of numbers 0-5 represent the the outer edges of a transition (ie: the farm texture; terr0003 blends into the grass texture; terr0001) The vertical row 0-5 represent the inner part of the transition.
0 1 2 3 4 5 1| x 6 7 8 9
2| x x 10 11 12
3| x x x 13 14
4| x x x x 15
5| x x x x x
See my explanation in this
thread about coastlines.