Just like economy class, business-class and first-class tickets come in multiple fare classes, and lower fare classes come with restrictions (such as not being

Just like economy class, business-class and first-class tickets come in multiple fare classes, and lower fare classes come with restrictions (such as not being refundable or requiring an additional fee to make changes). Typically for US airlines, fare class "F" is the unrestricted/"full fare" for first class and is fully refundable and can be freely changed; fare class "J" is the same for business class; and fare class "Y" is the same for economy class. United uses two codes for first class depending on the cabin arrangement. In a two-cabin (i.e., economy and first class cabins) aircraft, "F2" is the "full fare" first-class, while on a three-cabin (economy, business and first class) aircraft, it's "F3". There is also a "pecking order" attached to the fare class which affects how and when passengers get shuffled around in case there aren't enough seats in the cabin of that fare class. The highest pecking order is, as expected, from an "F3" ticket; next in line is an "A3" (discounted three-cabin first class), then "F2", then "A2", then "J", then the lower-tier business class fares ("C", "D", "Z" and "P"), and then the full-fare economy class "Y", followed by a mix of other economy fares and "upgrade" fare classes (the "upgrade" fares are ones representing a ticket booked into one cabin but with a condition, or points/instrument attached, which grants a space-permitting right to be seated in a cabin of higher class).