Parnel is correct. [Gawd that hurt to write!] Manufacturers assign a production line number and position to each plane in each order and to each option. An option ensures that there is an aircraft slotted into the line but unlike an order a full deposit is not required. At a certain date the airline must confirm its desire to turn this into a firm contract for delivery, and make a deposit payment. Since we are talking several years down the road, an option can be cancelled and the next option or ordered aircraft [i.e. orders that come in years later] will take its place.

This is a way for airlines to ensure delivery dates long into the future, but also at a certain point cancel aircraft they may not need. There is a cost to options, but penalties are usually waved, and they are less than the deposit required for a firm order.

It is likely WestJet will pull some or all of its options for a fair number of 737s, but will wait until closer to confirmation date to see if business turns around. From the last reports it has taken delivery of about one-third of its total 737 order, but at least half of the 90+ jets are options according to one airline analyst's assessment of the carrier.