The "custom folder" at the world map is
a feature of Moondust Engine and TheXTech: it's a similar directory to level's data directories, but it holds resources of the world map itself. This concept was created since the foundation of the Moondust Project to allow episodes to be more organised, so, you will don't need to flood the episode directory with custom resources of the world map itself. But, this feature is exclusive to Moondust Engine and TheXTech, and
it doesn't support by other SMBX branches such as X2 or SMBX-38A. So, when you targeting your world map for X2 or SMBX-38A, don't use this feature as it is basically absent here.
So, if you making an episode for TheXTech, this structure is allowed:
-
worlds/
--- (your episode name)/
----- block-x.png (shared between all levels of episode)
----- The world.wldx
-----
The world/
------- tile-x.png, scene-x.png, ... (custom resources of the world map)
----- Level.lvlx
-----
Level/
------- block-x.png, background-x.png, ...
In addition, both Moondust Engine and TheXTech allow you to have more than one world map in the same episode directory (each world map will be listed as a different episode in the list).
I.e.:
-
worlds/
--- (your episode name)/
----- block-x.png (shared between all levels of episode)
----- tile-x.png (shared between all world maps of the episode)
----- The world one.wldx
-----
The world one/
------- tile-x.png, scene-x.png, ... (custom resources of the world map one)
----- The world two.wldx
-----
The world two/
------- tile-x.png, scene-x.png, ... (custom resources of the world map two)
----- Level.lvlx
-----
Level/
------- block-x.png, background-x.png, ...
Trivia: I keep an idea to make the ability to have multiple different world maps in the part of the same episode and have the ability to travel between them like we do that between levels, but, this thing exists as a concept in my mind yet and no real attempts to implement this yet.