One of the most important rules is that flight segments must be used completely and in sequence with your ticket. All the airlines require passengers to travel as per the itinerary shown from the place of departure via any agreed connecting points till the final destination and vice versa. The airlines are selling a journey as a product, with a specific price attached to it each flight segment based on the market conditions. On any regularly scheduled flight, the passengers have paid a range of different fares, depending upon the tariff conditions under which they bought their tickets. This market segmentation is a critical element in the pricing of air travel and ensures that there are prices to suit every pocket and that seats do not generally go wasted.