What is an ADE Project File?
Sometimes folks get confused about the file format ADE uses. If you have been used to working with AFCAD or with AFX you will expect to work only with the bgl file. This is not the way ADE works
The bgl format is quite old and is specifically intended to store information about the simulator scenery. There are actually a lot of different types of format internal to a bgl file and that can be confusing especially when someone tries to open one in ADE or similar and can't Bgl files are good at storing scenery and airport information efficiently. These formats are proprietary and we do not officially know what is in them. There is no published file format we can work from. What we do know about the format is via 'reverse engineering' of the files themselves. Strictly this should not be done but so far Microsoft and others have not stopped us. Since we have no way to access the format we can't edit or change it. If we want some extra information then it is not easy to store.
It is not recommended to continually load and save from the bgl format. This can cause coordinate precision drift and some other problems. What does this mean - well if you keep opening up a bgl eventually things might start to move
Bgl files tend to contain different things so if you want to see the navaids, terrain and scenery all in the same place you would need to load several of them.
ADE Project Files
The project file is in a format specific to ADE. It contains information about both scenery and terrain. It also contains helpers such a guidelines and background images. There is no facility in the bgl format to store this sort of thing. The earliest project files have a .ade extension. There are some with .ad2 and the current format is .ad3 ADE can read any of these but will always save in the latest version (.ad3)
The simulator does not understand ADE project files so you do need to compile them to bgl files before they can be used. It is possible to re-construct some parts of a project file from the compiled files but it is usually not possible to do it all. It is not possible to get terrain data back for FSX and P3D since we do not understand the format. ADE project files do not suffer from precision drift.
So the 'rule' is to always save and load your work in the project file format. This will ensure that you do have all the information.