Are you talking about 2d screen points that you want to unproject out into 3d world vectors like this Project a 2D screen point to 3D world point and How to set 3d geometry position by 2d screen pos? Also check out the source code for picking to see how to convert 2d mouse points into 3d rays.
If not, could you give more information about what you are trying to do or demonstrate with a live example?