What does YC, XY and XT on the receipt mean?

Anybody has some clue about what these terms mean on a NW ticket receipt?
e.g. on a BWI-SYD award ticket
YC $5.00
XY $7.00
XT $117.05

I assume that XT means tax but could not figure out the other two.

YC is probably US Customs fee
XY is probably US Immigration fee
XT is probably the rest of the taxes/fees (probably including US 9/11 fee, USDA APHIS fee, US passenger facility charge, US Int'l Departure and Arrival Taxes, Australian Passenger Movement charge, Australian Noise Levy Tax, Australian Int'l passenger service charge, maybe transit taxes for any other countries you're transiting, too)

That's based on the breakdown of taxes on a BWI-SYD ticket on ITAsoftware.

They are all taxes:

YC = Customs Fee
XY = US Immigration/Naturalization Fee
XT = Combination Tax - i.e. XA (Agriculture), XF (Passenger Facility Charge),
US (Transportation Tax), AY (Security Fee) ZP (Flight Segment Tax)

Note: Combination Tax is used because I believe a fare ladder on a ticket only support 3 different tax entries but Uncle Sam (and in your case the Australian government) collect more than 3 different types of taxes.
SYD, for example, collects Noise Levy Taxes and Passenger Service Charges, while the Australian government collects an Air Passenger Ticket Levy and a Goods and Services Tax. I'm thinking the XT your paying is probably a combination of 5 or 6 different taxes.
And if you really care about taxes on your ticket, go to www.iata.org

Thanks a lot!

Based on the information provided, it looks that NW did NOT charge me $5.00 phone ticketing fee (I booked this tickets a few weeks ago). Wow, a deal!!!!

That fee would not be included in the fare basis of the ticket. Like Travelocity/Orbitz/Expedia, it would be a separate charge.