The core is the glued-together layer that fits into the door frame. It will be wallpaper glued to a thin plywood veneer glued to a large slab of styrofoam glued to the fiberglass exterior panel. This core would be slid into the three-sided frame of the door and then the 4th frame edge would be welded on. If you look at your bunkend, you'll see how the corners are welded together. That's how it would be done at the factory, anyway. Core with frame just adds the frame section for you, already welded. Now, instead of having to weld the door frame together, you'll just have to attach the piano hinge to the camper with screws, rivets, or both. IMO welding on the end of the door frame is probably easier than attaching the piano hinge while trying to balance the weight of the door assembly.