There are two messages to change the Flight Level: RFL and ECL. However, I don't know the differences. Can anyone explain them?
RFL is Requested Flight Level, ECL is the En-Route Cruise Level. RFL the cruising flight level that has been requested by the flight plan or by the pilots, ECL is the level which has been actually assigned by the controller. The two may be the same or they may be different.
From the EUROCONTROL SPECIFICATION FOR TRAJECTORY PREDICTION Edition: 1.0, page 33
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.
This is a complicated way of saying the requested flight level is usually assigned unless there's a good reason for the controller not to (to avoid potential conflicts for example), and the pilots can request a different flight level en route.