Six Characters

I wonder why they didn't go with the IMF Special Drawing Right, which is used in many other international contexts. (Including aviation - the liability limits under the Montreal Convention are in SDRs.)

There used to be a story going around, possibly apocryphal, that the process to reset them used to be entirely manual and generally unknown to the rest of the organization. There was a big problem the week the person who had that task retired. Someone around here was probably there if it really happened and can add some color.

IATA-registered airlines - it seems there are 370,

https://www.iata.org/en/about/members/airline-list/

The IATA has 367 active airlines.

Bear in mind that this doesn't apply to charter airlines, only public passenger ones.

Given there are about 200 countries in the world, you'd need 5 large airlines per country, which is a lot! Most of them don't have any and rely on other countries. Still more have a single national carrier.

Also, not every airline has a 3-digit code. e.g. Aero Republica has the two-alphanum designator P5, but doesn't have a 3-digit.

Two-letter codes are assigned to anybody on request, but three-digit codes are assigned only to full IATA members.

The three-digit code is used primarily for ticketing (it's the first three digits of a ticket number), and as an airline you only really need it if you're going to do complex interop things like ticketing another airline's flights. Most low cost carriers like Ryanair are not IATA members, and even Southwest only joined last year.

I suspect it's this part:

"This ticket was priced in GBP, not INR. Because the journey originated in Manchester, the fare is denominated in the currency of the origin country: the United Kingdom."

So: the currency is the one for the country of origin.