By way of background, FlightSim (both FS9 and FSX) displays objects by determining for each pixel on your screen the distance from your "eye" to the point on each object in view that pixel represents. Hence, the closest object is displayed fully while only those portions of objects further away that are not covered by closer objects are seen.
But, if, for example, you have created a building and a sign mounted on it as separate objects (so as to get higher resolution for the sign), both objects will be the same distance from "your eye" and this results in Z-fighting (i.e. "flickering"). To resolve such a situation, you move the sign a short distance (a few inches/cm.) away from the side of the building.
A similar situation exists when you attempt to cover the ground with, for example, a plane (not an airplane, a flat polygon) to be textured as asphalt as the base for a parking lot. If the parking lot is away from where the user aircraft might venture, you can place that plane slightly above the ground to eliminate flicker. The further away the plane can been seen, the higher it must be elevated because, when viewed at shallow angles, the benefits of its elevation are reduced (think trigonometry). But, if that plane is right beside an apron, its elevation, unless very small, will be apparent when viewed from the user aircraft on that apron.
As well, you cannot "draw" (place planes) directly on top of runways, taxiways or aprons; you must elevate them. Elevating only a mm/fraction of an inch works fine up close, but you'll get flicker when viewed from a distance. If you elevate enough to eliminate flicker, that elevation will be readily apparent up close.
FS8/FS2002 provided a mechanism to place planes directly on the ground ("ground-polys" for short) without causing flickering, As well, planes could be placed directly on top of runways, taxiways and aprons. Fortunately, while neither FS9 nor FSX have a similar "native" capability, they will display FS8 ground-polys. Unfortunately, creating ground-polys with Gmax was a tedious and time-consuming process.
But, with ADE_GP, it's a "breeze".