From ASSS Wiki
In SubSpace, each Zone consists of one or more arenas with a set of players in each. Each arena has a map and settings associated with it, though this map and these settings are not necessarily unique to each arena. Arenas can either be "Public" or "Private". A Public arena is the default arena where the server places entering players. Public arenas usually share the same map. A Private arena is accessed by players using the ?go command and usually has a different map or different settings than the public arenas. See Custom Settings and Misc Settings to configure how a private arena will save its scores (as the biller does not save these).
An arena's settings determine how many players can be in it. If a public arena fills up, the server will create a new duplicate for entering players. If a private arena fills up, the server will simply not allow new players to enter it.
A private arena is also hidden if its name starts with '#'. Hidden arenas will not show up on the arena list accessed through a client (?arena) for normal players (they will for staff), but any player can enter them with the ?go command. This allows a kind of arena access control without requiring code or configuration on the server.