A world of the challenge's own.
A challenge can bring its world: a base shape (the game's own, a flat world, a void, or shaped terrain with its height, hills, and roughness), its blocks, its biomes, and a platform to start on. It is made when the challenge starts and goes when it stops.
Transforms.
On top of the base, transforms run on each chunk in order: replace kinds of block with others, randomize everything, lay layers, a skygrid, a strip a few blocks wide, a cut below or above a height. They stack, so one wide and random blocks are two transforms apart.
Settings beside it.
Difficulty, the time and whether it passes, the weather, the world border, PvP, and game rules apply to the challenge's world, or to the server's own when there is no generator.