What are the child requirements for booster mode?

For booster mode, your child must weigh 18-54.4 kg (40-120 lbs), be 112-145 cm (44-57 in.) in height with the tops of their ears at or below the headrest top, and be at least 4 years old. Once your child exceeds any of these size requirements, you must discontinue use of this seat and your child may be ready for the vehicle belt system only. (Page 5)