Minecraft wiki entry for Custom_dimension, associated text: The root tag.
 type: The namespaced ID of the dimension type. Can be preset minecraft:overworld, minecraft:overworld_caves, minecraft:the_nether, minecraft:the_end, or a custom dimension type (configure via JSON files, see dimension type section) .
 generator: Generation settings used for that dimension.
 type (string): The ID of the generator. minecraft: followed by flat, noise or debug.
Other compounds (see below)