Overflow servers are created when the zone or world you're attempting to join is full. They're basically just another zone instance that's made. When you try to join a world/zone that's full, you will automatically be placed in an overflow server. You can earn XP and other rewards as normal on an overflow server, and it will consider you "in queue" for the "main" zone and once space opens up, it'll ask you if you want to join the main zone.
You will receive 3 options when the main server is ready to accept you:
- Travel - This will take you to the main server
- Reenter queue - This will allow you to remain on the overflow server, placing you back into the queue. You will then receive the popup again when there is room on main.
- Leave queue - This will allow you to remain on the overflow server without constantly receiving a popup to ask if you would like to stay.
Additionally, if you let the timer on the popup run out before you make a choice, it will default to "Reenter queue", leaving you on the overflow server, which will give you a chance to leave later.