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.