Inside X-Plane it has relatively little effect:
- It is one of many elements that counts toward the land area that will be flattened. (Runways and taxiways are also used.)
- We do not actually render any special terrain or fences.
- If an airport has an airport boundary, we do not calculate the airport's boundary ourselves - instead f we use the specified boundary.
- All land intersecting the airport boundary is turned to airport terrain.
- The DEM is flattened within the airport boundary to reduce the slope of high frequency bumps.
- The inner ring is closest to runways and taxiways (very close) with very little simplification. It is filled in with land if it is wet.
- A second ring slightly farther from runways and taxiways is also filled in with land if it is wet.
- The outermost ring is a lot farther out, but does not fill in Water. This is the ring that the airport boundary can replace.
Since the airport boundary polygon provides only one ring, it cannot be used for this purpose. At some point in the future, we might use the airport boundary for rings 2 and 3, or use a smaller version of the airport polygon for rings 1 and 2.
For now, my recommendation is: the airport boundary should trace out the entire airport premesis, not including water.