CITROËN DS 5 Handbook page 176

IUF: seat suitable for the installation of an Isofix
Universal seat, Forward-facing secured
using the upper strap secured to the upper
ring on ISOFIX seating positions.
IL-SU: seat suitable for the installation of an
Isofix Semi-Universal seat either:
- rearward facing fitted with an upper strap
or a support leg,
- forward facing fitted with a support leg,
- a sleeper cot fitted with an upper strap or a
support leg.
For advice on securing of the upper strap, refer
to the "Isofix mountings" section.
X: seat not suitable for the installation of a child
seat or shell for the weight group indicated.
Remove and stow the head restraint
before installing a child seat with a
backrest on a passenger seat. Refit the
head restraint once the child seat has
been removed.