A Group 1/2/3 (or combination i-Size) seat is designed to adapt as your child grows: full harness for the toddler years, then converting to a high-back booster that uses the car’s own seatbelt once they’re big enough, typically from around age 4 upward.
The transition from harness to booster isn’t about age alone — it’s about height and the child’s ability to sit still and keep a seatbelt correctly positioned. Moving up too early, just because a birthday passed, is one of the more common avoidable mistakes.
When comparing seats in this category, check ease of harness adjustment (some let you adjust height without unthreading straps), and whether the seat can be used in booster mode without the harness at all, since not every model in this group converts.