PURE SERVER SETTING
This is a new feature in the multiplayer setup screen. If the pure server option is "ON," then all clients must have the identical .pak file the server has. This was implemented to prevent cheating and hacking. Setting up a server with this off will enable any pak file configuration to enter the game. The .pak file is a very large compressed file that contains all of the games assets. Users can uncompress the file using the program WinZip.
But it has the potential in allowing some players to implement cheats available to them, I've seen them in practice, and they are cheats unless all players know it's available and have the equal opportunity to use it. Imagine having a radar screen on your HUD, and able to see where enemy players are behind walls and doors? You're chasing an enemy flag carrier, get to a door only to meet with an exploding rocket fired a second before the door even opened. Now it's a great tool if all the players have it, and the playing field is level.
Running sv_pure 1 protects you, the gamers, from hacked pack files and cheats that are capable via pack files. The sv_pure option works like this: if it's turned off, any pak file is visible that the client has; with sv_pure 1, however, only the pak files located on the game server are allowed on that particular server, so if you want your favorite HUD replacement available to you in game see if the server admin will load it to their baseq3 directory. Otherwise the custom HUD is rendered useless. However players with the HUD can still join and play. Since the latest point release you no longer will get kicked back to the main menu if you have other pak files; that was a bug in the code that id fixed.
One way to properly deal with this entire issue, is to make the server pure and make available to all players the special paks you will allow them to use. If you like a particular custom HUD and sound effects, put it on your web site for download. The other is to review and install submitted pak files from your player community. (side note: Please be aware, if you're an admin, you'll need to verify that the client pack files submitted to you are legit, and not pak file that may have a cheat in it.)