Minecraft wiki entry for worldborder, associated text: Specifies the damage a player takes per second per block past the world border buffer. For example, if <damagePerBlock> is 0.1, a player 5 blocks outside the world border buffer takes 0.5 damage per second (damage less than half a heart might not change the visual health display, but still accumulates). Initially set to 0.2.
Must be a Single-precision floating-point format number. And it must be greater than or equal to 0.0.