Kim et al.(2005) study a special AS/RS system that is similar to the man-on-board system in the sense that it picks multiple items from the rack in each cycle.The difference is that after each pick,the picked item must be put into a drop buffer that is vertically below the picking location.The routing problem for this system can also be formulated as a special TSP,and an x-coordinate based heuristic and a clustering based heuristic are proposed to solve it.