Yes sir, i understand your query that csab round is special round for josaa counselling as i would lile to give you a brief about it as follows:

JEE counselling was done through JAB and CSAB. Admissions to various IITs were made through Joint Admission Board (JAB) whereas Central Seat Allocation Board (CSAB) is responsible for intake in various NITs, IIITs, and GFTIs. After 2015 MHRD set up JoSAA to manage JEE counselling. The purpose of JoSAA counselling is that the admission to 97 institutes (IITs, NITs and IIITs, CFTIs) should be made through a common platform. JEE Main 2018 counselling and seat allotment is not conducted separately but is collectively conducted through JoSAA 2018 for 31 NITs, 23 IIITs and 20 GFTIs, which are the participating institutes of JEE Main 2018. Those candidates who are successfully allotted seats will have to pay the seat acceptance fee and verify their documents at the designated Reporting Centres. In JEE Main counselling section we are providing the basic information about JoSAA counselling, some important points to know while filling the registration form for JEE counselling, JEE counselling schedule chart, Brief about eligibility, Freeze, float, and slide options, Withdrawal of seats, Brief about all the different rounds (1 to 7),Supernumerary seats for Union territories, Brief about dual reporting, Institutes that participate in the counselling round IITs, NITs, Private institutes, GFTIs.
Please ask me in comments for further query