From FlightGear wiki
Found some resources:
- The Conference Survey. Published by Helios Information Services Ltd. Retrieved October 9, 2010. - Results of an online survey about CPDLC and its use prior to the ATN 2006 conference organized by Helios Information Services Ltd. (ATN = Aeronautical Telecommunications Network)
- ICAO Doc 4444 Air Procedures for Air Navigation Services-Air Traffic Management (PANS-ATM), Sixteenth Edition, 2016 (PDF). Retrieved October 9, 2010. - Very likely of interest are Section 14.3 - Exchange of Operational CPDLC Messages and Appendix 5 - Controller-Pilot Data Link Communications (CPDLC) Message Set.