You can optionally set the owner of actors and User Widgets when they are created. If the actor that contains the 3D widget is not assigned the correct owner, the client will have no authority to change anything. If the client should not own the actor, (ex. a unit in an RTS game) an RPC on something the player does own is necessary.
Note: this is a macro that is called by a multicast event that is called once for each player designed to create necessary UI elements for each player. The branch statement determines if the player is the correct one, otherwise, each player would have n UI’s where n is the number of players.