There are four kinds of air tickets, which are commonly known. First and Business class tickets, which are usually the most expensive, are “open”. This means you can change your flight times. Full-fare Economy tickets are also open. Cheaper tickets are usually “restricted”. You cannot change tickets with restriction on them. The best-known kind of restricted ticket is APEX (Advance Purchase Excursion). Often you have to stay overnight. This is something most business travellers do not want to do.