In Air Traffic Control their is quite a few acronyms for levels at various stages in the flight. I am not sure what you meant by messages, or more importantly which messages, so I have kept the answer a little broader.
Requested Flight Level
The level that is requested in the flight plan before the flight. There can be multiple RFLs in a flight plan. Each time the aircraft wants to cruise at a different level, in the flight plan, an amended RFL is added.
The Pilot may also request an amended RFL mid flight but the RFL is not mentioned at the time just the request for a different level.
Speedbird 1 Request Climb FL360
Cleared Flight Level
This is the level that the aircraft is currently cleared at or cleared to.
This can be a level on the way to the RFL used as
paperstop level for separation (a level the pilot is never expected to maintain but provides separation until conflict no longer exists);
The RFL when the aircraft is maintaining; or
Any other level that the aircraft is cleared to.
Enroute Cruising Level
Stole part of this one from the question: What is the difference between RFL and ECL in Air Traffic Control?, Thanks GgD for the resource.
From the EUROCONTROL SPECIFICATION FOR TRAJECTORY PREDICTION Edition: 1.0
Enroute Cruising Level - The level that the flight is to maintain for a significant part of the flight after reaching TOC and prior to TOD.
2.3.2 Enroute Cruising Level (ECL) (3.3.2)
Domain Knowledge: The ECL is a notional
level that, in the absence of constraints, equates to the
RFL . In the case where a flight is not permitted to cruise
at its RFL for any or a part of its route, due to the
presence of a strategic constraint (level limit) which applies to
flights between the aerodromes of departure and destination, the ECL
will be limited to the level defined by the constraint. The ECL may
also be changed by instruction from ATC where the flight is to
maintain a different cruising level, typically due to the proximity
of a flight at the RFL on the same route. As with the RFL ,
the ECL can be changed for different segments of the route and,
if different in the cruise phase, is changed to the XFL on
passing the sector boundary. The notion of ECL ceases to exist
after the final TOD from cruising level.
So summarising the above passages, the ECL is the level that will be maintained from top of climb until top of descent. The ECL can be the same as the RFL if the aircraft can be cleared to fly the RFL.
So if there is a conflict at the level of the RFL and the aircraft is cleared to a level below the RFL, the ECL would be that level.
When the aircraft is cleared to fly the ECL the CFL and ECL are the same. So you can get the RFL, ECL and CFL as the same level.
Personally I have never heard of this level acronym so I am unable to respond on its relationship to the others. If someone else knows I would love to know also.
Additional Level Acronyms
I am also adding the major other level acronyms for ATC into the mix so they are all together. So the answer is as complete as I can get it.
Exit Flight Level
This is the level that the aircraft is going to cross an ATC sector boundary at. There are two different ways of using the XFL around the world:
In some ANSPs (Air Navigation Service Providers) it is the level that the aircraft will be cleared at when the aircraft calls the next sector. In this case the aircraft could be on Climb or on Descent to the XFL or could be maintaining the XFL all ready. This is used so that the controller knows the CFL when the aircraft calls.
In other ANSPs is is the level that the aircraft will cross the boundary regardless of the CFL issued at the time. This one is used for trajectory calculations inside the receiving ATC system.
Pilot Reported Level
This is the level the Pilot last reported at. In Procedural Environments this is the only way there is to find out the current level of the Aircraft.
Mode Charlie Level
There can be other names and acronyms for this level but they all have the same meaning.
This is the level received by the SSR radar or by the ADSB receiver. This has the the aircraft's current level usually embeded in a message with other details. This can be used to fill in the PRL on some systems or is a complete independent system from the PRL.
Sorry I found this question so late but thought it deserved an answer anyway.