They should never be used with just a lap belt, Heppner stressed. A lap belt won't hold the child securely. Some older designs still on the market don't have high backs. The shorter booster seats use a shield and a lap belt goes over it to keep the seat and child in. The child could be ejected.
Many booster seats are actually two seats in one.
They have a harness so that the seat can be used as a child car seat until the 40-pound cutoff. The harness is then removed and the seat becomes a booster. That saves parents the expense of buying two separate seats. Getting a child who has been riding in a child safety seat to move up to a booster seat is usually no problem. School-age children are tuned into what their friends think and they often view a booster seat as babyish.