CSAB or Central Seat Allocation Board is the conducting body of spot round or special round counselling to fill the vacant seat which are leftover after JoSAA counselling is completed. CSAB conducts spot round counselling only for NIT+ system and IITs are not part of this counselling. In order to participate in this counselling you will have to participate by registering on its official portal after JoSAA counselling gets over. If you couldn't get seat upto seventh round of JoSAA counselling then you can apply for CSAB spot round counselling. You can read the full details, eligibility requirements and procedure by clicking on the following link:

https://engineering.careers360.com/articles/csab-special-round-for-nits-iiits-and-gftis/amp

You can predict your college and branch options based on your rank by using the JEE Main college predictor tool by clicking on the following link:

https://engineering.careers360.com/jee-main-college-predictor?icn=QnA&ici=qna_answer

Hope it helps you