What is the difference between RFL and ECL in Air Traffic Control?
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.