Transitional and Preliminary are two different one year PGY1s that are required for certain specialties.
Prelim is general ed for a more specialized field. i.e. dermatology may require a preliminary year in internal medicine. Urology may require a preliminary year in general surgery.
Transitional is like M3, except you get paid. Some do transitional if they still aren't sure, some do it because they can get away with doing this instead of prelim. Considered easier than preliminary, I think.
(But I'm a long way away, so if anyone wants to correct, feel free!)
Also, why is the 25th percentile for plastics from IMGs so friggin low?