If it's something a player would see in the game, it must be downloaded by that player before he or she can play, usually contained in a hak file (or sometimes in the NWN/override folder). Game files for anything a player sees or uses are not transmitted by the server to the player, they are instead read by the game directly from the player's hard drive.
You can have a hak that is pretty much empty for players that a builder can put module resources into server-side, but such a hak is limited in what it can contain: scripts, blueprints, 2da files that are only read by a module script, and a couple other limited use files.
Anything used by both the server and player client, such as 2da files for classes, skills, and spells, or tileset walkmesh files, must be the same in a hak on both the server and player computers. If they don't match, you could have crashing issues.