A preliminary year is in a specific field, say prelim surgery or prelim IM.
A transitional year features electives and rotations through different departments and can usually be taiklored by the individual. They are usually more competitive.
An intersting distinction many do not think about is transitional years are separately accredited and the PGY year you spend in the transitional year is funded by medicare separately from any future training thereby not decreasing your total number of funded PGY years available.
