bolt ins are just that...plates welded to the floor, cage bolted to them, cage bolted together inside car.
weld in....usually means cuttin holes in the floor, so u can weld the tops of the tubes etc, in the car, then welding the cage together in the car, from top down, then when the cage is all welded together, its lifted up, and the holes u cut previously are usually boxed and welded back up, with the cage then being welded to them.