Minecraft wiki entry for Processor_list, associated text:  heightmap: The heightmap to use. Can be WORLD_SURFACE, WORLD_SURFACE_WG, OCEAN_FLOOR, or OCEAN_FLOOR_WG. Default is WORLD_SURFACE_WG.
 offset: Offset of how far above or below the resulting surface the structure should be generated. Default is 0, while terrain-matching uses -1, which causes the structure to be embedded in the ground.