What are the child requirements for booster mode?

Children must weigh 18–54.4 kg (40–120 lbs), measure 112–144 cm (44–57 in), have the tops of their ears at or below the top of the booster seat headrest, and be at least 4 years old. In booster mode, only the vehicle's lap and shoulder belt system may be used to restrain the child — never a lap belt alone. Once your child exceeds any of these size requirements, you must discontinue use of the seat. (Page 15)