This is the standard inventory control system for the vast majority of small retail and manufacturing operations and is very simple in application.
The key manager will stand in the middle of the store or manufacturing area and looks around. If he or she happens to notice that some items are out of stock, they are reordered.
In retailing, the difficulty with the eyeball system is that a particularly good item may be out of stock for some time before anyone notices. Throughout the time it is out of stock, sales are being lost on it.
Similarly, in a small manufacturing operation, low stocks of some particularly critical items may not be noticed until there are none left. Then production suffers until the supply of that part can be replenished.
Typically, DHL will use a computerised system, which in turn creates a perpetual inventory system.