Companies have TWO systems for marking PSI levels on the side of a tire. Most use the "Max PSI" method. The PSI marked on the tire is the MAXIMUM safe pressure for that tire on a rim of the proper size and type that is in good condition...the maximum PSI is far higher than most lighter riders would ever want to use.

The second method is to provide a RANGE...the tire will be marked "90 PSI to 110 PSI". The lower number is the suggested PSI for a rider of average weight. The higher number is the maximum SAFE PSI level.

As a practical matter, many riders can "cheat" and ride at PSI levels higher than the "safety" rating. But, it is silly to do so...the tire will provide its best shock absorption, best cornering, and best braking at much lower PSI levels.