Advisories). Message Categories Via the AFTN the following message categories are submitted: distress messages; urgency messages; flight safety messages; meteorological messages; flight regularity messages; aeronautical information services (AIS) messages; aeronautical administrative messages; service messages. Priority Indicators Priority Indicators consist of two letters SS, DD, FF, GG and KK. They are assigned depending on the messages category as follows: Priority Indicator SS for Distress Messages Priority Indicator DD for Urgency Messages Priority Indicator FF for Flight Safety Messages Priority Indicator GG for Meteorological Messages, Flight Regularity Messages and Aeronautical Information Services Messages Priority Indicator KK for Aeronautical Administrative Messages Priority Indicator used for Service Messages are assigned as considered appropriate by the originator, but most likely KK is used Order of Priority The Priority Indicator is used to transmit AFTN messages according to their Order of Priority. So messages with Priority Indicator SS have the highest transmission priority. Messages with Priority Indicator DD and FF have the second highest transmission priority and the remaining messages with Priority Indicator GG and KK the lowest. See also References External links ICAO Aviation Data Service (ICAO official site) World Airport (ICAO official site) Aircraft operations Encodings Navigational aids Further reading List of aeronautical location indicators. Aviation communications International telecommunications