This is the best explanation I can find.I may be reading that wrong, but normally the pressure cap would NOT be on the expansion tank, but on the engine or water heater tank. The higher one I would think. Expansion tanks usually have just a silly splash prevention cap.
They use the terms header / expansion tank, and coolant recovery tank.
Again, though, they agree that the former is pressurised - although on the interweb you see the term used for both.