Logging different aircraft types
I'm not a spotter but, if "a friend" was keeping a list of aircraft types flown what would you all count as different types?
Would a 737-200 be different from a 737-300 as pretty much different aircraft?
Would a 737-300 be different from a 737-400 as variants of the same type?
How about DC9/MD80/B717?
F28 vs F100?
Then there are light aircraft C150, C152 are fairly close, but with different names, PA28-160 is fairly different from PA28-161 to fly.
How do you all define a new type?