Dear Prabhat, Neet mop up round happens after first and second round all india counselling.

All those candidates who have not got a seat in any college of their choice in earlier rounds may apply for this.

Based on your marks and ranking you will be allotted the left seats.

It will occur in online mode.

It will also happen for state quota and aiims for left over seats.

However most of the state counselling mop up rounds are offline but in aiims it is online and happens after 3 rounds of counselling.

All the best 

Hope this helps