Tileset Documention

I was looking at the tileset template and other the tilesets done by various people and was left wondering:

Is there any documentation as to what each id is supposed to be?

For example, what in the world is “animation_line” ?

A lot of tilesets seem to have this set to be the same as “line_trail” but, for the life of me, I can’t find any documentation (I did some searches of the source code but came up empty as well - I guess my search-fu isn’t that great) as to what it is meant to be.

Any help would be appreciated!

Most everything else is actually, more or less, self-explanatory but having some sort of documentation as to what this stuff is meant to be would be eminently helpful.