With many networks, including the UK and French systems, a contrast is offered between advance-purchase tickets, which do guarantee a (specific) seat and are therefore often non-refundable, and "walk-on" tickets purchased on the day of travel, which do not; these passengers may be forced to stand or take a tip-up seat in a vestibule.[19][20] Rail networks face less pressure in this than airlines, which cannot allow passengers to stand. Rail networks accordingly often do not have a centralised booking system; as passengers can stand, tickets can be sold from automated machines and clerks with no knowledge of how many people intend to board a train. In addition, rail networks have to deal with the unpredictable nature of season ticket holders, who have purchased a right to make unlimited use of a route; these passengers may often be allowed to join any train but not guaranteed a seat.[21][22]