How do I check if my flight is not a flight that only allow transit passengers? [closed]

Since the Taiwan High Speed Rail started operating in 2007, cutting travel times between Taipei and Kaohsiung to just over 90 minutes, all domestic flights between Taipei-Songshan (TSA) and Kaohsiung (KHH) have been terminated due to lack of demand.

The only flight connection left is on China Airlines between Taipei-Taoyuan (TPE), Taipei's international airport, and Kaohsiung. This is a "domestic-international" flight that can only be boarded after you have passed through immigration, meaning that you must have a connection out of the country at the other end. Since Taipei's airport is much larger, virtually all passengers would be going KHH-TPE-somewhere or vice versa.

This kind of thing most commonly occurs when an airline operates a route without fifth freedom rights: for example, foreign airlines are not allowed to transport domestic passengers in the US, meaning any flights they operate within the US are for transit passengers only.

None of this really explains the scenario in the TV program though, as if they were flying out of Kaohsiung to Tokyo, they should have been able to pass through immigration in Taipei. There's likely some critical detail lost in transmission here: for example, maybe the TPE-KHH flight arrives in the evening but the Tokyo connection was only in the morning, which is not allowed since KHH airport closes at night?

In any case, the airline or agency that sold the ticket was at fault, since this is not a legal connection. However, their liability is likely limited to either a full refund, or arranging alternative transportation.

Wikipedia's article on Kaohsiung International Airport appears to claim that China Airlines flights between Taipei and Kaohsiung are operated as international flights as far as immigration is concerned. This suggests that there may be some special rules in effect about this pair of airports.

Indeed, Google Flights refuses to show any direct flights TPE-KHH, but will show connections NRT-TPE-KHH as well as TPE-KHH-MNL. So it seems like the flights between TPE and KHH are somehow forbidden for purely domestic travel.

Perhaps the Taiwanese authorities are concerned about them being used as "virtual" visa runs, or perhaps the airline doesn't want to deal with domestic travelers who thought they wouldn't need a passport (or additional entries on their visa) to take a "domestic" flight. Or the airline may be forbidden from carrying domestic passengers to/from TPE due to congestion.

I agree with Aganju's comment that the airline would not sell you a ticket for a particular journey unless it is one they allow passengers actually making. The case you summarize must either be garbled somehow or be based on a misunderstanding.

In this particular case, flying from Taipei to Tokyo via Kaohsiung is a bit of a detour, so one hypothesis could be that it never became clear to the airline employee the passenger spoke to that she would actually be connecting internationally in Kaohsiung.