Look at the pdf manual, page#15 SHOULDER HEIGHT MARKERS AND THE NEXT SEAT ...
你要我看15页,15页说的是什么呢?
Next type of restraint that can be used in the car for your child.
When your child has outgrown the seat, you can then either use:
Use a booster seat suitable for children up to 8-10 years of age and a Child Harness suitable for use with the booster seat.
Or you can use a booster seat suitable for children up to 8-10 years of age with the car's seatbelt.
If a child's legs do not hang over the edge of the vehicle seat then they need to use a booster seat so that the seatbelt fits correctly and so the child does not slump. A lap seatbelt on the child's abdomen is dangerous.