Not sure but I think it is a subsidiary mainly for domestic flts. I have seen at different Japanese Airports 737 (I think 500er) only with Air Nippon as title.
Of course it would also make sense for them to put on all acft those titels since the word Nippon (Nihon) is not well know outside of Japan (or Nippon/Nihon).
EL ANK 768 AIR NIPPON
ANK means as far as I know All Nippon Link but in CRS flts from KIX to TPE are loaded under EL. Means they take them also on the Taiwan (ROC) flts since JAL has EG (Japan Asia Airways - JAA)
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