Minecraft wiki entry for Pre-flattening, associated text: Each map item's damage value corresponds to the map ID saved in each world's NBT data. The map's parameters, such as the scale of the map, its location in the world and what areas are explored are saved in separate .dat files, unlike written books or fireworks which store their additional data directly into the item itself. If a map is cloned they will both share the same damage value and newly explored areas will appear on both instances.