Can someone help with this? Feel so confused (car seat related)

[jklod]

www.silvercrossbaby.com/media/yxydir2p/simplicity-infant-carrier-with-simplifix-isofix-base-vehicle-application-list.pdf

Link here

what does an ‘ I size vehicle’ mean? Should I avoid a vehicle with that label?

any help massively appreciated

I-size seat is just a newer safer version. I-size vehicle = i-size approved. As explained here.

I don't think you have to avoid them per se. Just find your car's make/model on the list of vehicles in the document you attached and see if that car seat can be fitted in your car?