0 0 RESERVED
1 1 ATS (AIR TRAFFIC SERVICE) UNIT SERVING FIR (FLIGHT INFORMATION REGION)
2 2 FIR (FLIGHT INFORMATION REGION)
3 3 UIR (UPPER FLIGHT INFORMATION REGION)
4 4 CTA (CONTROL AREA)
5 5 VAAC (VOLCANIC ASH ADVISORY CENTRE)
6 6 MWO (METEOROLOGICAL WATCH OFFICE) ISSUING SIGMET
15 15 MISSING VALUE
