Minecraft wiki entry for Development_versions, associated text: New customization GUI:
Comes with 7 presets: Classic Flat, Tunnelers' Dream, Water World, Overworld, Snowy Kingdom, Bottomless Pit (Broken),[7] and Desert.
Custom starts off with Classic Flat with three options: Add Layer, Edit Layer, Remove Layer. Adding and editing are not yet included.
Platers can customize superflat without add/edit layer option using the "seed" in the preset list.