This was something I was bored enough to experiment with and learn a little more about. Okay, it was more like I had to since I'm writing a Tile Substitution / Placement script that has since become published. Some of the information here is probably a no-brainer to those who worked with autotiles before.
Each autotile contains 48 (ranges from 0..47) different tiles which appear in different directions. I have put together a list that explains all 48 of them should you ever want to use this information in some way. Here they are:
Autotile Tile IDs
If you're familiar with RPG Maker XP, you'd know that each tileset has a blank tile at the very top left of the tileset and then 7 autotiles the user can specify to the right of it. Here's a quick list containing tile IDs for the blank tile and each autotile in the tileset.
Now you're probably wondering, "okay, now what?" So say you need to call them or change them through the $game_map.data[x, y, i] = value call and you need to use an autotile for some reason. See, autotiles can't shape themselves, so you need to find the tile ID that best suits where you're putting a particular autotile. Now if you can't be bothered to remember all of the tile IDs and such (and honestly, I can't say I blame you because I wouldn't either), then you can use a quick formula I just made up to use an autotile should you ever want to:
(48 * autotile) + index
autotile: 0: Blank, 1-7: Autotiles #1-7
index: Autotile Tile Index (0..47) (refer to list above for directions of edges)