YQ can be anything the airlines want to charge. Many airlines put the fuel charge in the calculation line and fare ladder. Other airlines put it in the tax line.
no rhyme or reason
Small example:
BKK NRT rt.
TG charges the YQ 60.00 rt
UA charges 25.00 Q each way in the fare calc line.
JL charges 6.40 YQ rt
SQ charges 54.00 YQ rt
NH charges 6.40 YQ rt
- here is the fun part NH codeshares with TG and UA on that route (as well as their own flights) so one knew ahead which airline has the lowest tax that is the airline to go with- free ticket-wise that is as each airline has arious different pricing on that route -if you have to pay for it.