Isn't the more common usage of this in skyboxes? You'd have one area where the gameplay is taking place, then another area in the map that shows the larger scale area surrounding where the player is and the portal makes it appear that the gameplay area is within the other.

The designer needs to confine the player so they can't break the illusion (where Prey lets Morgan Yu shatter that 4th wall), or you can do things like put the player on a speeding train traveling through a scenery and falling off is fatal