It is the last and final round to fill vacant seats.Special Vacant Seats Filling Rounds, also known as special rounds are being conducted by Central Seat Allocation Board (CSAB) to fill the vacant seats at NITs, IIEST, IIITs and other GFTIs (Except IITs) after completion of all the JoSAA Rounds.The registration will start from 24th july.

Refer this link for complete info. https://engineering.careers360.com/articles/csab-special-round-for-nits-iiits-and-gftis

Hope this helps.