Opel 2016 ASTRA Owner's Manual page 63

IL : suitable for particular ISOFIX restraint systems of the 'specific-vehicle', 'restricted' or 'semi-universal' categories.
The ISOFIX restraint system must be approved for the specific vehicle type.
IUF : suitable for ISOFIX forward-facing child restraint systems of universal category approved for use in this weight class.
X : no ISOFIX child restraint system approved in this weight class.
ISOFIX size class and seat device
A – ISO/F3 : forward-facing child restraint system for children of maximum size in the weight class 9 to 18 kg
B – ISO/F2 : forward-facing child restraint system for smaller children in the weight class 9 to 18 kg
B1 – ISO/F2X : forward-facing child restraint system for smaller children in the weight class 9 to 18 kg
C – ISO/R3 : rear-facing child restraint system for children of maximum size in the weight class up to 18 kg
D – ISO/R2 : rear-facing child restraint system for smaller children in the weight class up to 18 kg
E – ISO/R1 : rear-facing child restraint system for young children in the weight class up to 13 kg
Seats, restraints 61