This is the FDA
which must be placed on any and all products containing the artificial
Sufferers of the metabolic disorder PKU are unable to metabolize the amino acid phenylalanine, which occurs naturally in a wide variety of foods. However, since aspartame is chiefly composed of phenylalanine, it would be quite bad for someone with PKU to consume diet coke or a similar aspartame-sweetened product. Unless you happen to have this fairly rare disorder, you probably don't need to worry about this label (except in regards to Parkinson's disease, and that is quite possibly just hot air anyway).
If you have PKU, you would have known it since your birth, since it's part of the routine bloodwork done on all infants.