Those are actually on the help file... maybe not in a very good explanation but the help file does have information on how each tilesheet is formatted...
Really, I couldn't find pixel sizes or even how each A is suppose to be used.
Basically all it says is A tilesets are autotiles, and it states some useless babble of how the system "behind the scenes" classifies the tilesets. Seeing how we don't have access to the engine code anyway, I fail to see how such is useful information.
Who gives a crap about how the system categorizes a feature behind the scenes. The help file gives you all the info you don't need, but basic things, like what "each" A does, and what size pixels(in some cases) the tile needs to be for X tileset format, good luck finding that info. If the info is there at all, It's buried really deep.
The same goes for every aspect you lookup in the help file,
To Caustic: Yea reading help files for any code, is the utmost worst way to learn coding(It's not just with Ruby either, It's this way for any code/engine with help files), because in help files, everything is a fragmented mess of who the bloody hell actually thought this would offer any help.
Every single keyworld or whatnot(everything in general is described separately) is described on its own page, so It's pretty much impossible to learn that way.
You're better off watching/reading tutorials+working with already finished code, so you can easily learn how it all ties together, and you get a full picture of what each aspect of the code is for, It's not a million, and one keywords each on its own stand alone page.