Over the years, States have changes helmet laws to include the requirement to have a reflective decal on the rear of the helmet. Not all states require this and the only reference that I can recall is seeing an AMA fold-out that listed helmet laws by each state. I can't find anything online about reflective decals on helmets.
I recently upgraded my Bell Qualifier DLX MIPS, (which I love) to a Bell Star. With the new helmet, a new decal needed to be found. I found the perfect one that has significance to the part of the country that I live in.
Fred