Minecraft wiki entry for Structure_Void, associated text: Structure voids can be placed as part of a structure and are ignored when the structure file is saved. This results in the blocks at the location where the structure is placed being maintained and not being overridden. For example, if a player saves a 2 block high structure with air at the top and a structure void at the bottom, and loads it into a 2 block high area of stone, the top block becomes air but the bottom stays stone.