Today most messaging is via typical email systems but still a proportion of inter personal airline messaging consists of Type B. Either the type B messages are via SITATEX or embedded in host airline systems as Type B.
Type B was unique to the Air Transport industry and consisted of a 7 letter address line.
Take the example LHRLLAA:
The first 3 letters e.g. LHR referenced the location (Heathrow Airport ).
The next two letters referenced a department, e.g LL (Baggage Tracing)
The final two letters referenced the Airline e.g. AA (American Airlines)