WME provides basic means of running games using normal screen aspect ratio on wide screen monitors and vice versa, without image distortions.
If the engine detects that the game aspect ratio is different from monitor aspect ratio, the startup settings dialog will contain a check-box titled "Maintain aspect ratio". When checked, the game will not use the resolution defined in ProjectMan, but a nearest higher "aspect ratio friendly" resolution. The game viewport will be displayed in the center of the screen and the remaining space will be filled with black.
The whole point is to prevent image distortions caused by different aspect ratios. Some people believe this should be handled entirely by video card drivers, but since current drivers are far from perfect in this respect, this feature is natively supported by the engine.
Please note, that WME does not differentiate between different wide screen aspect ratios. For example both 16:9 and 16:10 ratios are treated the same. This feature is really only intended for compensating between normal aspect and wide aspect scenarios. Also, the game viewport is never stretched or shrunk. It remains exactly the same size as you defined in ProjectMan.
There is one more related feature, requested by some WME users - the ability to use desktop resolution. In this case, the game will use the same resolution as user's desktop, the game viewport will be centered on screen, and the remaining space will be filled with black. It's equivalent to running the game in windowed mode, only in this case the window frame and other open windows are hidden, which is preferred by some players. The reasoning behind this feature is to allow the players to use native resolution of LCD monitors.
The "Use desktop resolution" option is disabled by default. If you want to make it available to your players, enable this option in project settings in ProjectMan.