The designer-provided data comes from an .INI file in the scenery own \SCENERY folder.
However, it can be customized normally in GSX because, as soon as you start making edits and APPLY the changes, a .INI file will appear in your %APPDATA%\Virtuali\GSX folder, which contains all the settings made by the scenery developer in the supplied .INI, augmented with your own settings. Your own .INI file will now take the precedence over the designer provided.
The advantage of having an .INI file in the scenery own folder, is that if you for any reason lose or want to reset your own .INI file in %APPDATA%, it will revert back to the designer-provided .INI, which will surely be much better than having to start from scratch, as if the airport were bare and not customized at all.
This is confirmed to working. For example, we supply .INI file with almost all our sceneries, so they all read "designer provided data", but you can surely customized them further. I just made a test, to be sure, and it works.
- Go to a scenery with designer-provided data, the GSX window will indicate only designer provided data is available
- Change any option, for example "Disable stairs". The "Apply" button becomes active.
- Click "Apply", the 2nd line above will now indicate a new .INI file has been created in %APPDATA%\Virtuali\GSX.