What's the difference between a trainee and a drafted/rookie player?
The difference between drafted rookie players and trainee players is that trainee player are not registered to play in matches until the teams registered them as official players. I think teams have to register trainee players as official players by the end of Round 3 or before the start of Round 4. Plus, trainee players are given a much lower salary too.
Some players in recent years who were trainee players and have become official players are IBK Altos PARK Min-Ji, Hyundai E&C JEON Ha-Ri, and Heungkuk HYEON Moo-Lin.