You can see the current superflat code of the preset in the code field at the top. When you first start the app, the default 'Classic Flat' preset will be loaded. And once you're done, you can simply copy the code into Minecraft to create some awesome and unique superflat worlds. Using Superflat Generator, however, you can customize every single aspect of it.
All you can do is choose from a set of a preselected codes and remove some of their layers.
Unfortunately, there is no documentation on how to write such a code, and in-game customization is very restricted as of now. By using a preset code this map type is highly customizable. Besides normally generated maps, Minecraft offers the option to create so-called superflat maps.
