I don't know if all child seats have the capability to utilise a top strap, but the ones bought by my offspring for the grandkids all have the facility. The strap is detachable and clips on at top rear of the seat, and at the bottom brackets I described. As you say, the Lap & Diagonal also goes around the seat forming the actual child restraint and holding the seat in place.However (not actually knowing), I assume that seats are available somewhere with the restraint straps built into the seat, or some other combination of construction which means that it is advisable to have the extra security of the top strap. In my case no effort is too much for the safety of the grandkids, so Iso, L&D and top strap all get put on when the seats are transferred into the B-Max.
In my motor , there is a diagram on the back of the rear seats just above the bottom bracketsshowing how to use them.
Geri
Edited by: Geriatricus