hello, 

the two rounds are for those who are getting colleges like if you get college of your choice in the first round you can withdraw and get admission to the college of your choice. 

if you dont, apply for 2nd round. 

and those who didnot get any college in both the rounds can check for the mop up round.

as simple as that.

refer to this neet 2018 counselling article for more info 

https://medicine.careers360.com/articles/neet-counselling

Thanks for the question.

If you have not paid the one time registration fees of the respected college, then you just have to withdraw your name and apply in the state counselling whenever it begins.

But its advisable not to withdraw a seat. 

Good luck