The inserts allow the upper section of the lift to act as a mezzanine for storage of other equipment, or to protect cars underneath when used primarily as a storage lift. The inserts can hold 800 lbs. if you were to drive right down the center. The inserts are gusseted underneath.
The HD models take (3) AD-46 inserts to enclose the center, and (4) AD-46 inserts for the XLT, WD, and each half of the SXS. There may be some space remaining at one end, but a drip tray or the jack tray can fill this gap.