Can I use both the LATCH system and the vehicle seat belt at the same time?
No, you must never use both the vehicle seat belt and the LATCH strap at the same time when securing the car seat. Both installation methods are equally safe, but only one should be used at a time. (Page 10, Page 25)
From the official Graco Graco Nautilus 65 Manual Booster Seat User manual, page 10
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