These ratings are meant to help parents decide which ones to buy. It can be a difficult task, since there are no federal standards to address how well booster seats are designed so that seatbelts, which are made for adults, fit properly on children who are too big for child safety seats.
Booster seats are meant to fill the gap between the time a child is too big for a forward-facing child safety seat with its internal harnesses, yet is too small for a vehicle's seat belts to fit correctly. Adult seat belts will not fit some children properly until about the age of 12. Booster seats raise toddlers and older children to help correctly position a vehicle's lap and shoulder belts, so that children are properly restrained in a crash.