g142437141 wrote an answer about Maxxis Larsen TT Mountain Bike Tire on October 2, 2008
It is the durometer, the lower the number the tackier the rubber compound. Although the down side of tackier rubber is that it wears quicker on the road and slickrock. My advice is go with the exc, 60a, or 62a. Exc is a maxxis race compound.