This supplement concerns child restraint systems comprised of a base fixture to which a portable shell can be attached. The base fixture is attached to the vehicle by the ISOFIX anchorages in the seating position and the shell attached to the base. However, since the shells may also be designed for use without the base fixture in a configuration where the shell is secured by the adult safety belt of the seating position, this supplement would enable type approvals under UN R129 for this configuration.