Pinky
Could your instructors be refering to to difference between a twin that requires a class rating (MEP (Land)) and a larger more complex twin that requires a separate type rating? If you obtain a MEP class rating then you will be able to fly any aircraft of in that class, whereas a type rating will restrict you to that type.
The list of aircraft requiring a class or a type rating are in AMC FCL 1.215, but I don't have a copy at home. I can get hold of a copy when I next go to the airport if you are unable to get hold of one.
Airclues
www.jaa.nl/jar/jar/toc0572.htm