Fares are calculated in "units" called NUCs. NUCs are based on the US dollar and is the standard form of fare caluclation.

The Q5.00 is a tax based on the city / airpory which carries a tax. However, this is already caluclated into the fare and is not a surcharge. The alphabets XY etc after a number differentiates the tax from the total tax which is shown as at XT on your ticket on the lower left hand corner just below the fare total. As you can see, there are only so many lines available for fares plus tax so any extra taxes are grouped together under XT and then seperated out on the fare construction.

Yes, I was a travel agent ...