The "Inside Glass" option changes the printing process so that you can apply your decal or cling inside a window with the graphic facing outside. This protects the image from any outside elements that could damage or wear on it.
In general, "Inside Glass" is strongly-encouraged with the Static Cling or Clear Static Cling materials because they do not have adhesive and could be removed by strong weather or vandalism.
This is also a great option when concerned about building or leasing requirements. Business signage installed on the outside of a window is sometimes considered a “banner” and may not be allowed by the property management company. The "Inside Glass" option is a much-safer option when trying to meet leasing requirements.