When should I use an extra cushion?

“PERO’s extra cushion must be used if your child weighs less than 15kg (33lbs) or is shorter than 80cm (31.5in).
This extra cushion is designed for children transitioning between Group 0 and Group 1 car seats. It increases your child’s height so that their head can properly rest on the headrest and provides much more comfort for your child’s back.”