Opel Automobile Owner's Manual page 48

48 Seats, restraints
U = Suitable for universal category restraint systems for use in this mass group, in conjunction with three-point seat belt.
< = Seat with ISOFIX mounting available. When mounting an ISOFIX child restraint system, only systems that have been
approved for the vehicle may be used.
X = Seat position not suitable for children of this mass group.