JA+ is a  server side Mod for  windows, linux, and MAC which has the possibility to get a Client side plugin with pk3 files.
this plugin is designed to give UI, animation and feature enhancement to your client side game.
You are not obligated to install the plugins with the JA+ server side part to run a JA+ server.

If the JA+ server you play is PURE server (sv_pure 1), all players and the server will have to install this plugins to join this server.
	so in this case the JA+ server is not server side anymore.

If the JA+ server you play is NOT PURE server (sv_pure 0), the server and players are not obligated to install this plugin to join the game.
	So in this case the JA+ server remains a server side mod.
	the server will recognize the players who have the plugins and will make them take advantage of it to have better animation, Ui and features.
	The players who have not this plugins will be able to join but will not have those better things (and they won't see the difference with a normal server side JA+ ).
	The fact that several players are running this plugin on a server where other players are not running it will not change and alter the gameplay of player.
	PS: in sv_pure 0, all pk3 at client side can be used by the client: SO if the client has to many pk3, he may have problem with missing textures or joining the server.