Todd - there is no difference, essentially you just have three trip meters.
The trip button by the instruments toggles round A, B and odometer. The FCD button toggles between whichever one (A, B and odometer) of the three you're on to the segment display. When you're on the segment display you have to hit the FCD button to go back to A, B or odometer.
The idea is basically that you can monitor say your tank average (trip A), a particular journey (trip B) and bits of your journey (segment) at the same time. The FCD button lets you toggle and reset the segment meter using one touch and without messing around sticking your hands between the steering wheel to reset or toggle the trip counter.
Took me a while to work this out. I also regularly try to adjust the mirrors using the FCD button then have a panic when nothing happens.
Resist - go easy, not everyone is as familiar with the car as you.