Although some brands and publishers use i-Size and R129 interchangeably, there are some differences between the two.
ECE R129 refers to the most recent and safest European safety regulation that controls the manufacturing of car seats. The ECE R129 safety regulation provides better side-impact protection, requires rear-facing for longer, and makes it easier to find the best-fitting car seats for your child.
i-Size is an enhancement of ECE R129, that makes it easier to confirm your i-Size car seat's compatibility with any i-Size approved vehicle - reducing the risk of installing your car seat incorrectly.The idea behind i-Size is that all car seats will eventually fit in all cars.
While all i-Size car seats meet ECE R129 testing standards, not every ECE R129 car seat fits i-Size vehicles compatible seating positions.