My google-fu should help.
AY - aviation security surcharge
Used for airport screening, sky marshalls, security training
$2.50 each flight leg, $10 limit per round trip
CJ - Amsterdam International Airport (AMS) Security Service Fee for Transit Passengers
€7.25 each way, €14.50 round trip
RN - Netherlands Passenger Service Charge for Transit Passenger
€6.33 each way, €12.66 round trip
US - domestic passenger ticket tax
Used for US aviation trust fund (FAA)
7.5% before taxes
XA - U.S. Animal and Plant Health Inspection Service (APHIS) Fee
Applies to all flights originating abroad and landing in the United States, Puerto Rico, or the U.S. Virgin Islands
$5.00
XF - passenger facility charge
Used for airport improvement projects
$4.50 per takeoff, $18 limit per round trip
XY - U.S. Immigration and Naturalization Fee
Applies to international arrivals to the United States, Puerto Rico, or the U.S. Virgin Islands
$7.00
YC - U.S. Customs User Fee
Applies to international arrivals to the United States, Puerto Rico, or the U.S. Virgin Islands
$5.50
YR - Carrier-Imposed Surcharges
Carrier-imposed surcharges stated separately from the base fare on some international itineraries
ZO - Denmark and Greenland: Passenger Service Charge