Actors which are created automatically when press Play

Hi all!
I create a empty level which does not have any actors. But when I press ‘play’, there are many actors appear in Scene Outliner : CameraActor, GameMode, GameNetworkManager, …
Could you explain to me about these actors? I cannot find any documents refer to them

Help me, pleaseeeeee

Hi, I am new for UE too, and I got the same problem now,

Have you solved it ?