Planar projection will project an image onto a surface as if you were projecting the image through a slide projector onto a wall. Planar Image Maps are best used on flat, or nearly flat surfaces like the sides of buildings and the screens of video monitors.
For the X and Y axes, Planar images are projected from the positive axis side of a surface towards the negative axis side. This means that the image appears correct when viewed from the positive side and it appears reversed if you view it from the negative side. For the Z axis, Planar images are projected from the negative side.
If you encounter this reversing effect and it isn’t what you want, you can reverse an image back by making the Scale value negative for the axis that needs to be reversed.