According to my manuals
The ICAO ILS clasification system consists of 3 characters (eg III/E/4)
1) Facitility performance category: I, II or III
2) limit point of course structure (ie point where localizer conforms to the required accuracy):
- point A: 4nm before threshold
- point B: 1050m (3500ft) before the threshold (cat I decision point)
- point C: glide path altitude of 100ft HAT (cat II deicsion point)
- point T: threshold
- point D: 900m (3000ft) beyond the threshold (cat III requirement only)
- point E: 600m (2000ft) beyond the runway end (cat III requirement only)
3) level of integrity and continuity of service 1,2,3 or 4 where 1 is the lowest and 4 the highest level
eg III/D/1 - requires us to operate to Cat I
HP