Hi Dipak, they are actually two categories present whoch are taken into account for most of the exams in India. Open category also called general category is the highest category. They are also known as forward caste and a rank obtained here is considered as all india rank. Generally for all admission and exams open category has the highest cutoffs. OBC which is called Other Backward Caste, is a caste which is lower than the Open category. It is the second highest category which has cutoffs not as AIR, but OBC category ranks. The cutoffs for these castes actually remain less than the general category cutoff but is higher than MBC, SC, ST category cutoffs.