Hello Student
Cut off score is the minimum possible score that candidates must achieve to qualify for AIIMS.
Examination authority, decides cut off marks based on different factors such as availability of seats, number of applicants who has applied and difficulty level in the entrance examination as well.
Based on cut off score, number of candidates will be called for the counseling
For more information about AIIMS, you can stick to this link
http://www.askiitians.com/medical-exams/aiims/