The Automation Engine messages file uc.msl contains seven types of messages. Particularly if one uses the AE Java Application Interface, one is likely to encounter messages of all types. Only a few message types are documented (in the v11 User Guide).
In an incident back in 2014, I asked Automic Support for a description of the other message types. Here is a summary of the response I received.
|C||Control — fixed messages and headlines|
|D||Control — messages for server|