As my understanding,
All UEs are members of one out of ten randomly allocated mobile populations, defined as Access Classes (AC) 0 to 9. The population number is stored in the SIM/USIM. In addition, UEs may be members of one or more out of 5 special categories (Access Classes 11 to 15), also held in the SIM/USIM. These are allocated to specific high priority users as follows. (The enumeration is not meant as a priority sequence):
Class 15 - PLMN Staff;
1) 14 - Emergency Services;
2) 13 - Public Utilities (e.g. water/gas suppliers);
3) 12 - Security Services;
4) 11 - For PLMN Use.
In all cases, the RRC establishment cause is set to ‘High Priority Access’ if the UE uses Access Class (AC)
11 to 15.