By default, the decals are designed to go on a painted surface or the OUTSIDE of a window.

But, you may want to choose the Reverse option for 2 reasons.

1.  The most common is if you have a graphic that you want to face the same direction on either side of a vehicle.  Note that if your design contains lettering or numbers this option should NOT be used as your lettering will appear backwards (so you would need a mirror to read it).  See the bottom image for an example of lettering that is Reversed.

2.  Another reason would be if you are wanting the decal to be installed on the INSIDE of a window for proper viewing on the outside.  Normally I do not recommend this option since it makes the decal harder to see when outside (the reflection on the glass will keep it from being seen).  Also, many of the premium color options (like reflective and colored chromes, etc.) aren't colored on the adhesive side so be sure to check the color chart for more details.  PRINTED "FULL COLOR" style decals CAN NOT BE USED ON THE INSIDE OF WINDOWS.  But sometimes you may need the decal to installed on the inside of a window in which you would use this option.