Hello, 

As per your question, i would like to tell you that the mop up round is only for the vacant seats left over after 1st and 2nd round of counseling. 

The main criteria fir mop up round is that the candidate has has to register themselves for mop up round and fill up the choices and the fee for mop up round. After the result, if the student qualify for the seat, they will be alloted the seat in college, and the student has to report the respective college within the given time. Mop up round is an offline process. 

For more details, check out- 

https://www.careers360.com/question-what-is-mop-up-round-in-neet