FOSS gamedev and creative worlds

In the menu exist a configure option for worlds, but I didn't found sth. about it reading the documentation and there is no example.
Is there any way that this can be used for new gamemodes?
I want to make for example a option that you can set the blocks to be generated for my flatland (already made this editable ingame with commands, but in menu would be better).

It says "nothing here yet" because he hasn't finished developing it, it's not functioning. I think it might be rules or map-editing.

Okay.