Minecraft wiki entry for World_border, associated text: When set to a negative number, the entire world is treated as outside the world border, and the border warning overlay appears throughout the world. No blocks can be mined and entities take damage unless they are within the border safe zone.
When set to NaN, the entire world is treated as outside the world border, but no border warning appears. No blocks can be mined, even in Creative mode, although entities do not take damage.
When set to infinity, the border occurs normally.