1) Selective rack
Steel storage racks arranged in either single or back-to-back (double) rows. Each of the rows offers a bay or number of bays in the down aisle direction. Pallets are generally positioned side-by-side in the down-aisle direction at each level of the bay. The advantage to this configuration of rack is that all pallets are accessible from the facing aisle.
2) Single deep selective rack
Rack structure which allows loads to be stored one unit deep on either side of an aisle.
3) Pallet flow rack
May be considered back-to-back selective rack. Inclined conveyor wheel sections are installed in the rack enabling back-loaded pallets are allowed to flow by gravity to the front or pick face. Pallet flow rack is useful for the movement of fast-moving items and serves to increase density of storage and throughput.