The letters "SL" on the sidewall of a truck tire indicate that the tire is a standard-load tire that holds a maximum load pressure of 35 pounds per square inch. This is the maximum load the tire carries, and manufacturers determine this capacity based on the size of the tire.
Manufacturers also use load ranges to separate tires that are the same size but that have different internal constructions that affects their strength. Tires with higher load ranges are constructed for added strength; they can carry more weight and hold more air pressure. Inversely, tires with low load ranges have a reduced capacity to carry weight and hold less air pressure.