Some countries issue "travel documents" that look like passports to certain non-citizens who for whatever reason cannot get a passport for their nationality (eg. they are refugees, claiming political asylum etc.).

The field "Nationality" would = Passenger's claimed nationality (sometimes it can be "stateless").

The field "Passport place of issue" would = country that issued that "travel document" - This tells the airline that the passenger can re-enter the country which issued the travel document.

Example. Refugee from country X fled to Canada to claim political asylum. He needs to travel to country Y to make a brief visit for personal reasons and cannot get a passport from country X. So he instead applies for a "travel document" from Canada and uses it to get a visa for country Y. The "travel document" would have a notation that he can "re-enter Canada". When using these types of "travel documents", visas need to applied for every country the holder would be visiting.