The Nintendo DS 3D hardware wants coordinates in the right-handed system, this is the positive x-axis points to the right, positive y-axis points up and the positive z-axis points toward you.
Direct3D on the other hand adopts the left-handed system, where the positive z-axis points away from you.
Since the goal of the N3D project is to create a very similar Direct3D API, N3D also adopts the left-handed cartesian coordinate system and converts the passed data from left-handed to right-handed coordinates under the hood. This is actually not much overhead, it basically scales the view matrix by -1.0 in the z-direction, voila.
Further information about 3-D Coordinate Systems can be found in the MSDN : http://msdn.microsoft.com/archive/default.asp?url=/archive/en-us/directx9_m/directx/direct3d/gettingstarted/3dcoordinatesystems/coordinatesystems.asp
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