Label 40 (UM40) messages are a user defined message type. They are uplinks from ground stations to the plane. According to the ICAO's Gold (Global Operational Data Link Document) 2nd edition and ICAO Doc 4444 it is a reserved message type.
They take the following forms atleast from my research and messages I've seen. 1. Free-text messages from the air traffic control or other ground stations to the flight crew. 2. Requests for weather updates or other information 3. Other operational or administrative messages
Some example messages for each message content
- Free Text discussing altitude changes with ATC.
FROM ATC
"SHOWING HIWC 15NM RADIUS
OF LOZ.
TOPS ARE AT FL300"
FROM Crew/Pilot (5Z message label as response)
"OS KMIA /FTM
WE CAN GO UP TO FL340
IF THAT LOOKS BETTER
FROM ATC
"THAT SHOULD BE GOOD
LGT ONCL MOD TURB
FL300-FL380"
- Requests for weather updates or other info
"WX RESPONSE
METAR
KFWA 140154Z 07014KT 10SM FEW250 22/14 A3012 RMK AO2 SLP196 T02170144
KFWA 132320Z 1400/1424 09007KT P6SM FEW250"
- Other operational or administrative messages
"REQUIRED/
XX - 01-LEFT ENGINE
02-RIGHT ENGINE
03-BOTH ENGINES
AUTOMATED FLT OPS MESSAGE"
"FLT 3991/16 ATW-CLT BAG E
GATE E14B
CREW CONNECTING GATE INFO
CREW SEAT FLT DEST GATE TIME
C F 1 2 3815BHM E14B 1930
END"
"MINIMUM FUEL 009933
EXTRA 02757 0052"
"CLEVELAND OH ................ B15 AA
HARRISBURG PA ................ C12 AA
LITTLE ROCK AR ................ E13
MIAMI FL ................ A4 AA
MYRTLE BEACH SC ................ D11 AA
NYC/JFK"