Tile autotiling
Tilemap Town supports sixteen different kinds of autotiling. Autotiling is a technique where a tile automatically changes its appearance according to what tiles are nearby. This makes it easier to create a map quickly, because instead of individually drawing variants of tiles with edges and corners on them, you can just draw "water" tiles and have the edges and corners appear automatically.
Tilemap Town expects the graphics on the tile sheet to be laid out in a specific way, according to the autotiling style that was chosen. The diagram above shows the different layouts that can be picked. The green squares are the tile you would actually choose when editing a tile.
The reason that there are autotiling layouts that are identical except for the "origin point" is so that if someone makes a client or a Tilemap Town map viewer that doesn't support autotiling, it can fall back to the tile that would make the most sense. For instance, a wall would probably be best with the fallback as the "no connections" tile, but a floor would probably be best with the "all connections" tile.
None
No autotiling; the graphic that was picked is the one that will be used.
9 slice
Basic autotiling where you provide a 3x3 square. Only really works for rectangles, though if you don't need tiles for horizontal or vertical sections (which will use the middle tile in that case) it may work for other uses too.
4 directions - Middle
Like 9-slice, but there are tiles provided for horizontal and vertical tiles, as well as tiles that are not connected to anything. The origin point is the "connected to everything" tile.
4 directions - Single
Same as above, but the origin is the "connected to nothing" tile.
RPG - Middle
Similar to the way autotiling works in RPG Maker 2000 and RPG Maker 2003. The origin is the "connections on all sides" tile. Unlike in RPG Maker, the top left tile is used when there are no connections. The top right is used to add inner corners, which is something that the "4 directions" styles don't support.
In this autotiling layout, the tile that gets drawn on the map is built up from four 8x8 pieces instead of using whole 16x16 tiles exclusively.
The tile at the top is not used and you can put anything you want in there.
RPG - Single
Same as above, but the origin is the "no connections" tile.
Horizontal - Middle 3
Simplified version of "4 directions" that only supports horizontal connections. Doesn't have a tile for when there are no connections.
Horizontal - Middle 4
Same as above, but there is a tile for when there are no connections.
Horizontal - Single 4
Same as above, but the tile for no connections is the origin.
Vertical - Middle 3
Simplified version of "4 directions" that only supports vertical connections. Doesn't have a tile for when there are no connections.
Vertical - Middle 4
Same as above, but there is a tile for when there are no connections.
Vertical - Single 4
Same as above, but the tile for no connections is the origin.
Quarter 8-way - Middle (8)
Vertical strip consisting of the "connections on all sides" tile, "connections on no sides" tile, inner corners, and the right, bottom, left, and top edges on a 16x16 tile together.
Like the "RPG" layouts, the tile that gets drawn on the map is built up from four 8x8 pieces.
Quarter 8-way - Single (8)
Same as above, but the origin point is the "connections on no sides" tile.
Quarter 8-way - Middle (16)
Similar to "Quarter 8-way - Middle (8)" except that you can provide a different graphic to be used for "side" pieces depending on what corner the piece will go on. This allows you to have a repeating 16x16 pattern, and add borders to the sides without interrupting the pattern.
Quarter 8-way - Single (16)
Same as above, but the origin point is the "connections on no sides" tile.
Other autotiling settings
In addition to the layout, you can specify an "autotiling class" and "autotiling edge class" which give you more control over how tiles connect to each other.
If a tile has a name, it will be connected to another tile if the second tile has the same name, or if the second tile's tile's autotile edge class matches the first tile's name. If a tile has an autotile class, the name will be ignored, and it will be connected to another tile if the second tile has the same autotile class, or if the second tile's autotile edge class matches the first tile's autotile class.
If you have an autotiled water tile with a grass border around the edges, grass tiles should act like they're connected to the water, but the water should not connect to the grass. Therefore the name or class would be specific to the water, but the edge class would be something grass would connect to.