Those candidates which leave seats even after first and second round of both state counselling and all India counselling are supposed to go to the students which attend mop up round . Only those students are eligible which have not been allotted ANY seat in AIQ and state counselling . So, what basically happens is a bunch of people gather in an auditorium and submit their application for mop up round .
Last year I attended mop up round for IMS BHU , idk why there were people present there which had 580–600 marks and they didn’t take any seat and decided to take seat in mop up round .(last year i had about 500 marks)