Wouldn't a holdout plane normally be used to put a transparent hole into the door to overlay it on a separate render, instead of cutting off everything around it? Because of the holdout wall, the shadows cast on the floor cut off in a straight line too. It's not that noticeable here because of the field's border but it would be in other footage.
As far as I understand it, the "Holdout Plane" hides everything behind it from the active camera's perspective in the rendering. The cutout for the door allows camera rays to hit the scene objects that are inside the door frame as seen from the camera's current position.