APlayerController::GetSeamlessTravelActorList

Called on client during seamless level transitions to get the list of Actors that should be moved into the new level PlayerControllers, Role < ROLE_Authority Actors, and any non-Actors that are inside an Actor that is in the list (i.e. Object.Outer == Actor in the list) are all automatically moved regardless of whether they're included here only dynamic actors in the PersistentLevel may be moved (this includes all actors spawned during gameplay) this is called for both parts of the transition because actors might change while in the middle (e.g. players might join or leave the game)

also GameModeBase::GetSeamlessTravelActorList() (the function that's called on servers)