Depends under what jurisdiction you hold a licence. Presuming JAA, Oz or similar: The licence you hold is the underlying authority about what you can and can't do. The class rating then extends those privileges onto a certain group of aircraft. The MEPL extends your CPL privileges to Multis. No different to the SEPL giving CPL privileges in aircraft that fit in the single engine piston land group. You could do MEPL first or instead of SEPL if you want.