Great question! Some booster seats have isofix (also known as latch) systems in them. These work differently to the isofix / latch systems seen on some convertible car seats or capsules.
If you have isofix points in your car, then you can use the isofix / latch system to secure the booster to the vehicle seat. This is great for when your kids jump out of the car as it keeps everything in place and your booster will not become a projectile if you suddenly stop. It also keeps the seat from shifting when your child is on it.
You will still use the vehicle seat belt as the primary point of connection with the vehicle, and to secure your child. With that in mind there's no weight limit to the latch system specifically as it isn't holding your child in, it is just securing the seat.
This contrasts with car seats for younger children. With a car seat designed for a younger child, like a convertible car seat with a harness or a capsule, commonly there is a weight limit on the isofix or latch install method. This is because, in those cases, the primary point of connection with the vehicle is the latch / isofix system. That system has a weight limit.
With seat belt boosters that come with the isofix / latch feature, it is safer to use BOTH the latch AND the seatbelt. This is because the two features do not interact with each other, and are performing different functions. The seatbelt is the primary point of connection. The isofix / latch is for stability of the seat, and security when the child is not in the seat. It is not the primary connection point.
With convertible harnessed seats, and with capsules, it will be less safe to use both features. This is because, the isofix / latch would be performing the function of the seatbelt. That is, it becomes the primary point of connection to the car. If both the latch and the seatbelt are threaded through the same part, having them both done up will mean you naturally get wear and tear and rub between the two. This could fray both belts, and mean your primary point of connection with the vehicle is compromised.
If the primary point of connection with the vehicle is compromised, what does that mean? It means the seat may not be secure in an accident. You are putting your child in a position which the manufacturer has not tested for. You are compromising your child's safety. Scary stuff, aye?
As a tech, I am always so grateful when people ask questions about things like this. It can be quite complex, especially if you don't work in car seats every day. If you have more questions for us, please reach out in the comments or via the chat function. We're here to help.
Boosters with Latch or Isofix Systems: