I hope this link helps, but it may also confuse you further. Basically till 6 months reversed, 6 months till 4 years in a seat with a built in harness, 4 years to 7 years in a booster with either a lap belt or H-harness.

A seat with a built in harness is only suitable if the child's head/shoulders are still sitting comfortably in the seat ie. the shoulder straps are passing over the childs shoulders at the correct angle and the childs head is not to far over the top of the seat. If it is a larger booster with a built in harness is required, they now have a couple of ones that have adjustable head rests to fix this problem.

Choosing a child restraint