I can comment on the anti-submarining part - it means the seat has a clip that goes between the legs that the seatbelt hooks into and this in effect stops the child from sliding downwards under the seatbelt, hard to explain - but it's a good thing!

Whereabouts are you...is it done by age or height when to transfer to a seatbelt - I know on our Safe n Sound Hi Liner it has marks to show that the shoulders should be above these or the child should stay in harness seat.