Fare classes may also vary by how far ahead the ticket must be purchased, or how long the length of stay is. For example, full fare economy class passengers are usually able to make changes to their reservation, while discount economy class passengers in the lowest booking code usually have tickets that are non-refundable, non-upgradeable, non-transferable, or non-changeable without a fee.
Airline fare classes are commonly indicated by letter codes, but the exact hierarchy and terms of these booking codes vary greatly from carrier to carrier.