This is referring to rear tires on a dually truck. Tire sidewalls, on tires that can be used singlely or in dual, will have two maximum weight ratings marked. One rating for single use and a lower rating for dual use. In the event that one of the dually tires has a failure, the remaining three rear tires will be able to support the load long enough to move, at a reduced speed, out of immediate danger. view full answer...