There are 2 phases , Real allotments and Spot allotments.
In real allotments the Candidates will be called on basis of there rank . Viz 0-1000 , 2000-5000 . and they will be allotted taking the fees and required certificates. after the real allotments if there are still seats they will be calling a spot admissions where the candidates will be picked from the crowd present at the venue according to the cat ranks.