Dear aspirant

You have not mentioned the name of entrance exam or the name of the counselling you are talking about. So I an answering you on general basis.

Mop up round of counselling generally happens when all the online counselling rounds are over but seats are still vacant . In this round you can rebuild and re sort your choice filling list and preferences. This is the final round of centralised counselling. If seats are still vacant then some of colleges go for spot decentralized counselling.

Best wishes.