Hello aspirant, 

NEET 2019 counselling will be conducted in two rounds for 15% All India Quota seats while for the Central & Deemed Universities and ESIC colleges, the counselling will be conducted in three rounds. Candidates who chose to participate in the second round of counselling at the time of reporting at the allotted college/university after the first round of counselling.

Mop up counselling in NEET 2019 is provided by states against reverted seats from All India 15% Quota. Basically, these are the seats left vacant after the first and second round of counselling. Those who do not get seats in either of the two rounds can participate in Mop Up rounds of NEET Counselling 2019.

Hope this helps you. 

Good luck.