The ISOFIX system secures the car seat using compatible connection points on the car and the seat, removing the risk of a poor fit when using the car's seatbelt to secure it.
Most cars now have ISOFIX points in the rear as standard; some also provide them in the front passenger seat (check your car's manual).
In Group 0+ and Group 1 seats, in particular, using the seatbelt rather than ISOFIX means running the belt along a specific path through and across the seat frame. Getting it wrong can introduce excess slack or a twisted belt, which could compromise your baby's safety.
The ISOFIX seats greatly reduces the risk of errors. A study by the German Insurance Institute showed that 95% of child seats using ISOFIX were fitted correctly, as opposed to 30% for seats attached using only the car's seatbelt. With the exception of two-seater cars, ISOFIX will be mandatory in all new models launched from November 2012, and in all vehicles manufactured after November 2014.
Some ISOFIX car seats feature indicators that confirm when the seat is correctly fitted. This is particularly useful with Group 0+ baby seats that are frequently removed from the car, but because it is theoretically possible for the indicators to fail, it's a good idea to check the seat manually too.
What are the downsides to ISOFIX?
One issue is expense. Some car seats can cost almost twice as much when fitted with integrated ISOFIX features. With some Group 0+ seats, you need to purchase an ISOFIX base (about 100) that is compatible with your model of seat; this attaches to the car's ISOFIX points, and your child seat attaches to the base. The base also features a leg which rests against the floor of the car for added support.
Some child seats come with an integrated leg and connections points, while some also require a top-tether that attaches to points on the back of the rear seat. That means ISOFIX child seats aren't always quicker to fit (a consideration if you frequently swap your seat between ISOFIX points but they do make a consistently safe fit much easier to achieve than with a seatbelt.
However, the biggest problem with the ISOFIX system is that, while the system was intended to be universal, not all ISOFIX seats and booster seat bases fit all cars with ISOFIX points. That's because child seats and the car's own seats can vary in shape, so they may not fit together properly, even though they share ISOFIX connections.