Yep, I could have been clearer I was asking about the surcharges part of the taxes and fuel surcharges addition to the base fare.

Japan's Ministry of Land, Infrastructure, Transport and Tourism regulates fuel surcharges for exJapan tickets based on actual fuel prices with airlines having to regularly apply for permission to charge at a given rate. JAL issues a press release every two months about it, e.g. most recently: https://press.jal.co.jp/en/release/202408/008257.html

What's murky is how this works in practice for xONEx tickets given the caps assume . I believe that's been discussed on flytertalk with (from memory) it varying based on ticketing carrier.