What is the difference between "User ID" and "ActorNR"

Friends, what is the difference between "User İD" and "ActorNR" in the "Photon" plugin

I can change "UserİD"

But how do I change "ActorNr" ?

When sending an event to a specific player in the TargetActors parameter, I write "ActorNr" of the player to whom I send the event (raise event)