Because a human being is no longer looking at a piece of mail and making sure it has correct postage before canceling it, there has to be some way for the postal equipment to determine if a letter is properly franked.
Collectors can see the tagging by using an shortwave or longwave ultraviolet light.
Fluorescent tagging is visible under longwave UV light , and does not leave an afterglow. It will glow only while it is under the light. Phosphorescent tagging glows under shortwave UV light, and does have an afterglow for a short time after the lighting is removed. Tagging may have been added in strips or blocks on a stamp, or over the entire stamp as a finish coating, or added to the fiber of the paper. There are several books that deal with the detailed history of tagged stamps, and the Scott U.S. Specialized Catalogue, for one, specifies tagging details for U.S. stamps.