What are the requirements for using the seat in booster mode?

In booster mode, the child must weigh 18–54.4 kg (40–120 lbs), be 112–145 cm (44–57 in.) tall, have the tops of their ears at or below the top of the headrest, and be at least 4 years old. Once the child exceeds any of these requirements, you must discontinue use of the seat entirely. (Page 55)