Mop up round is usually the last round of counselling in NEET for MBBS and BDS colleges. It is conducted in order to fill the vacancies, if any, in the medical as well as dental colleges (in states as well as for the all India seats). Since it is the ultimate round, candidates will no longer get a chance to secure a seat after the mop up round. So it is better that you take the seat allotted to you as you can not hold it in majority of the cases.