"I" car seats category is for kids who have from 9 to 18 kg. Small passenger can sit in forward-facing position now. But please remember - in this range You can also buy car seat that still allows to put baby in reward facing position (RWF) - this is 5 times safer. Thanks to this in front hits all impact is absorbing by baby's back, not very poor neck.
In "I" category are 5-point safety harness but in addition special torso-protection system - for maxiumu safety. But You must remember - if you drive a car and baby is in it, You have to drive carefully and lawfully.