When placing orders for new aircraft, airlines commonly obtain options from the aircraft manufacturer. These options allow the airline to delay the purchase of additional aircraft until market conditions become clearer and the purchase can be justified. It also reserves the airline a place in the manufacturing queue, for a guaranteed delivery slot. When the airline finally exercises its options, it can place its order without having to join the end of the queue which otherwise may delay the delivery of the aircraft for years. If future conditions do not justify expansion of the airline's fleet, the airline is not obliged to purchase the aircraft. An example of this is of an airline purchasing 30 planes up front and having options for an additional 20 for later delivery.
Rolling options differ from ordinary aircraft purchase options, in that the delivery slot is assigned at a later date, when the option is exercised or expires.