Common type ratings
i was looking through the FAA -PILOT CERTIFICATE AIRCRAFT TYPE DESIGNATIONS and have a question i cannot find the answer too.
i see that multiple aircraft models can have one type rating but im confused on how to make sense of when there are multiple aircraft models that have two type ratings for example:
the models ATR-42 and ATR-72 have a common type rating ATR-42 and ATR-72.
i read with common types that you can get a ATR-72 rating by completing the difference in training if you have a ATR-42 rating
so does this mean that if you first get rated for the ATR-72 then you can fly the ATR-42 and the ATR-72 or do you need both ratings if you want to fl both those aircrafts?