Hello, having just filled in an Advanced Passenger Information form for BA to fly between UK and Spain, the info not being taken correctly, my worrying about having to match it exactly to my passport, calling them up, etc. etc., I've tried out of pure curiosity to work out exactly what the API is *for*, especially since I am for the first time travelling as a dual national, which means I have had to make decisions about which travel documents to use that I've never had to make before.

So far, I've determined

1. It seems to be an outgrowth of one of the USA's responses to 9/11 that has been exported worldwide (like ETIAS seems to be a tit-for-tat response to ESTA).
2. It's obligatory, so you have to supply it no matter whether you understand it or not.
3. You're supposed to match exactly what's in your passport, but if you're checking in online then the passport isn't actually checked to make sure it's you nor that you've entered the correct information (obviously "don't be a dick" applies here, but it makes it weird as a security procedure).
4. It doesn't have to match the passport you're using at emigration barriers (for countries that have them) and immigration barriers - which would make sense, since dual nationals flying between both their countries are gonna use separate passports at each end.
5. As far as the airline's job, it it is to collect and submit API to the destination and probably origin countries' immigration authorities around 24h and possibly again 0h before the flight.
6. Those immigration authorities don't need to actively confirm to the carrier that each person on the list is allowed / not allowed to fly, i.e. the information passes one way.
7. It's likely that immigration authorities use the list to check for people who have a warrant for arrest, are suspected of whatever is defined as terrorism today, etc. <-- this is the only thing that I actually have found is the purpose of the whole exercise, but it seems to conflict with 3 (ETA: not 2).
8. It's separate from visa waiver travel authorisations (ESTA, future ETIAS, etc.) which will actually give you a yes/no result in advance as to whether travel is permitted.
9. It's separate from the check at security barriers that bring you into the departure lounge, since they compare your boarding pass with your passport, and the boarding pass is separate from the API.

These days I hardly fly at all, so I'm extremely out of date on the modern dance, but I am curious. What am I missing, if anything, please?