Client side and Server side

Hey I had a quick question about Client side and Server side relationships, I know that if you want the serverside to do a client side function you use usermessages
But how would i make client side do server side functions besides using concommands? As those lead to exploits.

Console commands are the only way of communicating from client to server, if you are worried about exploits, you should do some server side checks to ensure the player running the command isn’t doing anything malicious.