Dear aspirant

I am afraid to confirm you that " preparatory rank" is not really a rank . Every IIT has separate quota seats for SC , ST , OBC etc. These seats go vacant in every year . So the vacant seats are allotted to students of that respective category for a one year course . They are trained in the  IITs If they pass then they are eligible for undergraduate courses at IITs and they get direct admission in those . However students  doing preparatory courses can sit for JEE ADVANCED next year also.

Best wishes.