you can only travel between the overworld and personal space.
By using a Home Key, you are teleported to an empty dimension, 512 blocks from anyone else. when you use it again, you teleport back.
when you join the world, you are assigned an id, which determines where your island is. (starting at 0). your island will spawn at [0, 62, (id*512)]. and when you enter, it will always place you there. it will also put a bedrock block beneath you, and air where you are, to make sure you don't suffocate or fall into the void.
in the personal space, you start with only 1 bedrock below you and your inventory, but can bring blocks from the overworld.
