In AssaultCube 1.1 there are three different mastermodes. Mastermodes define the rules that are used when accepting players into the server. These three mastermodes are Open, Private, and Match. To change between these modes, a player may call a vote using the /mastermode command. Once all users disconnect, the server defaults to the open mastermode unless the server has a player password assigned to it.
An open mastermode will allow anyone and everyone into the server provided that they are not blacklisted or banned. Naturally, the server cannot connect more clients if it is already full. This is the most common mastermode. Public Servers (pubs) are always in this mode by default.
A private mastermode will not allow anyone to enter the server, even if the client was there previously. This is different than a server that requires a password to join. Anyone with an admin password may join even if the server is running in this mastermode.
A match mastermode is like private, except that someone who has left in the the middle of the match may rejoin at a later time. This is useful for clan matches and inters, because if someone accidentally disconnects, they can rejoin without needing to open the server up again. You cannot get AFK kicked when the server is in this mastermode. It also denies players from using the spectate function. This mode was added into 1.1.