Admission criteria includes Cat score. Different IIMs have different sectionwise and Overall CAT perentile Cutoffs.

Once you clear this cutoff, you get Calls of different IIMs to your registered Cat email id.

After that there are processes of GD-Pi-Wat. In which your skills are evaluated by the IIMs.

Now each process mentioned above has some weightage which differs from IIM to IIM. Like some IIMs give 50%+ weightage to Cat score and others might give 30% to 40% weightage for it.

Other criterias include workex, Gender diversity, profile diversity, overall profile including 10th 12th marks, grauation marks & PG marks if any, Reservation policy of govt. Etc. 

Once you are evaluated on all these parameters a merit list is prepared for each and every candidate. Suppose an IIM has 200 seats then top 200 people from that list are called for payment and blocking of the seat.

In case someone doesn't show up for the same, next candidate is called and so on.