From the Prepar3D Learning Center, Cockpit Basics:
"Pressure altitude is the altitude shown on the altimeter when the pressure is set to 29.92 inches of mercury (or 1012.2 millibars). Pressure altitude is important in calculating density altitude, a critical factor in determining aircraft performance, true airspeed, and true altitude. In the United States, aircraft fly at pressure altitudes or "flight levels" when operating at or above 18,000 feet MSL (5,486 meters). That's why you must set the altimeter to 29.92 whenever you fly at or above that altitude."
Best
Andreas
