The second control, Photo Set, let’s you pick which photographs to idealize in the project. If you only idealize some photos in a project, there will be two copies of the camera, the original camera for the unidealized images, and a new idealized camera (centered principal point, no lens distortion, etc.) for the idealized images.
Note that if self-calibration has been used in a project (so there are different sets of camera parameters per photo), Idealize will not be available.