Essentially, you can go from pistol to rifle, back to pistol again. If you build a pistol with a 16" barrel, you can have a QD buttstock on the 16" pistol and not create a SBR.
However, when the stock is removed, it is not functional unless you also reattach another QD stock harware, but I'd like to know if there is some sort of insert to put in the receiver to allow it to function with the stock off and no QD hardware attached to the pistol.