How does a proper client function work

It says “It executes on the client owning that actor”.

Given the case in almost every game the player pawn actually spawns on the server and the clients have a replicating simulated copy. As such who’s the owner over the pawn? the client or the server?

Since multicast functions called from server run on server as well , which is undesirable for cosmetic effects one would want to call a client function for spawning cosmetic effects, but how would that effect spawn on other clients if it only runs on the owning client?

how did UE3 / UDK managed multicast scenarios?