dear aditya sagar

i hope you what is a preparotory rank, it is basically seats left vacant because of the reserved seats left for the reserved category.

All the IITs runs preparatory courses of one year duration for reserved category candidates intended to prepare them to pursue the academic program (for session 2018-19) to which they have been allocated. At the end of this course, candidates are declared as pass or fail.
On successful completion of the course, the candidates will be offered direct admission in July 2019 (Academic Year 2019-20) to the already allotted undergraduate course.

i hope this was helpful.

GOOD LUCK!