The different projection systems used in geographical data might be one of the aspects of MSFS scenery design that most designers know hardly anything about. But it surely a very important thing.
Almost each country has its own projection system and although most of these systems will give you the position in either degrees or meters, that does not mean that it will all fit together. So when using data to design your scenery it is a good idea to take a look at which projection system they use.
In the terrain SDK it is written that the MSFS mesh terrain uses the WGS84 projection system. But an interesting question remains which projection is used for the scenery objects (so the coordinates in meters you define in the MDL object file). As objects are in general small, the influence of the projection system will be a small, but it would still be interesting to know.
While working on my Dutch sceneries I first met all the trouble that different projections can give. We got some data that used the Dutch Rijksdriehoekscoordinaten system (the position is defined in meters from a church tower near the center of the country). But compared to the WGS84 projection system, the Dutch RD system also has an additional rotation of a few degrees. So when you think you can just use the position in meters, you end up with your scenery in the wrong location. In the end we luckily found a set of good formulas to do all the transformations to WGS84 and back.
Another example where you will meet different projection systems is when you use a background image in a program like Ground2k4 or SceneGenX. The mesh scenery and the XML style scenery are all defined in WGS84, but a lot of maps are in meters. So in that case you would have to reproject the image to get a perfect match. SBuilder is a tool that has added this reprojection functionality. If you do not take notice of this, you might be positioning your scenery at the wrong position, especially when you get further from the reference point.
For those that have become really enthousiastic about projection systems after reading this, take a look at the Proj4 library. With this library you can easily convert between almost any projection system. At work I have also used this library to build some conversion tools.