The entire world of airline coding (as in "encoding") is fascinating to me: there are all kinds of ancient semi-human-readable formats like ACARS[1] and IATA Teletype[2] that are still pushing data around. Not the mention standard meal codes[3], like BLML for "bland meal" and CLML for "celebration cake meal."
[1]: https://en.wikipedia.org/wiki/ACARS
[2]: https://en.wikipedia.org/wiki/Airline_teletype_system
[3]: https://en.wikipedia.org/wiki/International_Air_Transport_As...