I can't find the link now, but I remember reading an article stating there are three levels to this thing. One, the obviously visible and known features, such

I can't find the link now, but I remember reading an article stating there are three levels to this thing. One, the obviously visible and known features, such that metallic strip many notes have. Know to both the users and would be counterfeiters. The know but not obviously features, such as micro text or the things that become visible when seen under those purple lights (UV?). Secret features only known to a very small select group of people in the money printing business. These are last resort identifiers of fakes.