The Speed Graphics have two separate exposure systems. The first is a lens mounted in a leaf shutter at the front side of the bellows. The second is a focal plane shutter at the back of the bellows. The focal plane shutter is a piece of rubberized fabric with slits that moves in front of the film to make the exposure. To use the focal plane shutter, you need to leave the front shutter open OR to use the front shutter you need to leave the focal plane shutter open. Crown Graphics do not have the focal plane shutter. Speed Graphics can use lenses that are not mounted in a shutter.
I see, that explain it very well even i didn't understand yet, but i have to check out what you've said to know/understand it.