I am sketching a ten-hour Narita connection on separate tickets, arriving at 07:20 and departing for Seoul at 17:45, and my checked bag may not be tagged through. I would feel uneasy booking until I know whether I must collect it, pass Japanese immigration, and leave enough time for a new check-in; I also need to know which passport, onward ticket, hotel or other documents to carry if the plan changes.
The decision turns on the airport process, not just the word “transit”: a through-checked bag and airside boarding pass may support one analysis, while baggage collection, landside check-in or a terminal change may require entry permission. Before paying, ask both airlines in writing whether the bag is tagged to the final destination, whether a boarding pass can be issued airside, and where immigration occurs; then compare the answer with the current Japan guidance. Checklist: passport, both booking references, onward ticket, accommodation or address if relevant, and a generous connection buffer. Exceptions include schedule changes, an overnight connection, or a different airport, so treat the airline confirmation as operational rather than a visa guarantee: https://www.mofa.go.jp/j_info/visit/visa/index.html.