FSE does not use the airport database from ANY specific simulator, nor any specific nation, nor any specific international agreement. The FSE Game World is a completely fictitious world, and any resemblance to real world airports, airport names, city names, country names, or otherwise, is completely coincidental. All users of all simulators must ensure that there is a takeoff or landing facility available within their simulator scenery that corresponds to the "FSE Airport Zone" within the fictitious FSE Game World, or make adjustments to their simulator as necessary, or ignore that particular airport.
To determine your current location in the simulated world, FSE takes your latitude and longitude coordinates, and finds the closest "airport center" datum coordinate. The closest airport centering mark to your airplane is where FSE considers your aircraft to be. This technique results in FSE having "Airport Zones" - some zone can be only a few square miles while other zones can be thousands of square miles .
Due to the way that location if FSE is calculated there are times when the close proximity of airports to each-other can cause issues in your simulated world. For a list of common "problem" airports, please see the dedicated page.
The airport exists in your simulator, but not FSE - Many airports in the "real world", or even in your simulator, may not exist in FSE. That does not mean that you cannot use this airport while playing FSE. FSE does not use the airport identifier (ICAO or country code) to determine your location, so you can very easily land and take off at any airport that exists in your simulator (including fictitious airports you may have created or downloaded), even though there is no corresponding airport in FSE at that exact location. Read the section below for more information about how the FSE Client determines your location for more on how you can land at a missing airport.
The airport exists in FSE, but not in your simulator - Many airports in FSE may exist in some simulators, but not others. That's ok - you can land there anyway. You just need to know the latitude and longitude of the FSE Airport, and takeoff or land at that geographical location within your simulator. Maybe there is a road there... or maybe you can create a runway using your simulator's airport creation tools.
Many real-world airport identifer codes do not match FSE identifier codes. If the FSE identifier code is not in your simulator, try searching based on the airport name or city/town. FSE does not use the identifier codes themselves to determine your location, so you can very easily land and take off at any airport in your simulator, whether or not there is a corresponding airport in FSE.
Requests to add new airports to the FSE Airport Database, or update FSE Airport information such as incorrect or outdated identifier codes, can be done by using the FSE Airport Suggestion Box.
Thanks to Teddii, HES18 and the X-Plane community on the FSE forums, all FSE airports missing from the X-Plane database are available for download as custom scenery. For download and installation instructions visit the X-Plane board.
Google Earth add-ons (kml) are available for download for both the FSE database and the FSX database. You can look at the airport in Google Earth and see both at the same time. You can even see if there is no FSX airport at all (such as Meigs Field in Chicago). The files can be downloaded from this page: Google Earth Files
In this example, the white dot is the FSE Airport Identifier Code, and the Red X is the FSX Identifier Code.