What are the requirements for using the booster seat mode?

For booster mode, the child must weigh 18 to 49.8 kg (40 to 110 lbs), be 112 to 145 cm (44 to 57 in.) tall with ears at or below the headrest top, and be at least 4 years old. The booster can only be used with lap and shoulder belt systems, never with lap belt only.