Older car seat regulations grouped seats by your child’s weight (Group 0, 0+, 1, 2, 3). i-Size (R129) instead groups by height, and requires rearward-facing travel until at least 15 months, plus mandatory side-impact protection testing that older seats never had to pass.
In practice this means an i-Size seat is generally a safer baseline than an older weight-based seat of the same price, and it removes some of the guesswork — you fit by your child’s height rather than trying to estimate which weight band they’re in.
i-Size seats are also tested and approved for a specific list of car models rather than being universally compatible, so always check your specific car is on the manufacturer’s approved list before buying, especially for older or less common cars.