Fare rules can restrict most any aspect of a journey. Often they restrict the passengers who may use the fare – limiting special discounts to children, for example -- and the travel agents who may sell the fare. Many fares include restrictions on the flight numbers, locations and departure times of flights within the fare’s fare component. Typically fares within the United States prohibit stops of longer than 4 hours within a FC. Fare rules may also impose restrictions at the priceable unit domain, such as the Saturday night stay restriction that depends on the times of flights from the first and last FC in the fare’s PU simultaneously. Rules very often restrict the fares that can combine in a priceable unit, such as requiring them to be on the same airline or have similar fare basis codes.
It is even possible for a fare to restrict parts of the journey outside the fare’s priceable unit. As will be shown, this greatly increases the difficulty of the search problem.
One passenger’s fares may restrict another’s, such as cheap companion fares that force a second or third passenger to accompany the first, and that restrict the fares those passengers may use to pay for common flights. This can cause an exponential increase in the complexity of search with the number of passengers on the trip.