I found the following description regarding the difference: "I am the Cybex Solution S I-Fix and differ from the Solution S-Fix by the i-Size standard. This standard ensures that much more is demanded of me in crash tests than of the normal S-Fix. The criterion in these tests is not the weight, but the size of the child. I can be used from the age of 3 until your child is about 12 years old. My patented headrest, which can be adjusted in height and inclination 12 times, prevents your child's head from tilting forward when he or she is asleep. This keeps your child's head in the safety zone even during an accident. With the integrated linear side impact protection, I offer your treasure the highest level of safety in the event of a side impact, because when it is folded out, it ensures that the forces acting on your child are reduced by up to 25%. My optimised ventilation system is particularly comfortable and ensures that the seat climate always remains optimal. I am connected to your car via the ISOFIX system, so I am even safer in the event of an accident."