Offstage actors are not rendered and do not collide, so they are effectively removed from the scene.
(Advanced) Note about hierarchies: when an actor moves on or offstage, it brings its entire subtree of descendants with it. Example: suppose you have a plane actor that's the parent of a wing actor that's the parent of a turbine actor. Then, if you move the plane offstage, the wing and the turbine will also go offstage with the plane. When you bring the plane back onstage, the wing and turbine will come with it. HOWEVER, if you move a child on/offstage independently of the parent, it will detach itself from the parent. In this example, if the plane starts out onstage and you move JUST the wing offstage, then the wing will detach from the plane and go offstage, resulting in a wingless plane left onstage, and the wing-with-turbine system offstage. If later you bring the wing back onstage, it will not automatically reattach to the plane, you must do that manually.