"Custom" is a world type that allow users to modify the generation of the Overworld, Nether, and End dimensions. It also supports the ability to create new, custom dimensions. It is an editable .json file that is imported ingame in the world creation screen.
You can find, fix, and complain about this open source code in it's github repository. If you run into edge-cases and problems, let us know - so we can all fix it! You can test some default builds by clicking on these links: default minecraft, custom superflat dimension, etc.