So im making a script that will fire an event and a local script that makes when the event is fired change the camera subject of all players but ¿Should I use FireClient or FireAllClients? (Sorry for my bad english)
Simply you should use FireAllClients() with an RemoteEvent if you want to do that. FireClient() requires an actual player parameter which is the difference between the two. Both have actual value arguments which means you can send the cframe of where you want to position the camera and more.
Remotes only fire to a client if they are connected to the RemoteEvent.OnClientEvent signal, it's the same thing with FireAllClients. Which basically means you can have a group of players that have different RemoteEvents that can get different client info, just to let you know!