The first character of the fare basis code is always a letter, and will almost always match the booking class.[3] Booking codes are the identifiers used by the airline's revenue management department to control how many seats can be sold at a particular fare level. For example, a plane may have 25 economy seats still available and the airline may show it in a reservation system as Y7 K5 M4 T6 E3 which indicates how many of each booking class can be reserved. Some codes cannot be sold by agents, and those seats may be reserved for international connections, loyalty programs, or airline staff relocation.