In the passenger airline case, capacity is regarded as fixed because changing what aircraft flies a certain service based on the demand is the exception rather than the rule. When the aircraft departs, the unsold seats cannot generate any revenue and thus can be said to have perished, or have spoiled. Airlines use specialized software to monitor how seats are reserved and react accordingly. There are various inventory controls such as a nested inventory system. For example, airlines can offer discounts on low-demand flights, where the flight will likely not sell out. When there is excess demand, the seats can be sold at a higher price.