Unreal Engine levels can be very large. At any time, a player might only see a small fraction of the total number of actors in a level. Most of the actors are not visible or audible, nor do they have a significant effect on the current player. The set of actors the server determines are capable of affecting a client in any significant way are considered relevant for this client. The set of relevant actors is determined on a per-client, or, in networking terms, a per-connection basis. Unreal Engine only replicates actors to a client if they are relevant for that client.