I agree that those specs can be clear as mud, and sometimes don't seem to relate to much. Lawn /garden tractors specs give the diameter of an uncut circle with a given mower size, and that seems clearer. The spec you are seeing is the turning radius of the tractor with / without brakes to the outside of the rear tire. So, in theory at least, if you turned as tight as possible on a surface you could see, you would make a circle 16.4 feet in diameter when measured to the outside tire track.