If I was to FireAllClients for something to appear or whatever then if another client (plr) joined a few seconds after the FireAllClients went off would they see it aswell or would it only effect clients that were in the game at the time.
No, it’s only the clients in the game at the time of firing the remote. If you want it to fire for new players as well you’d have to fire the remote manually each time a player joins.
It will only send to the clients that exist when the server sends FireAllClients.