Hi

My answer will be based on 2020 business rules as the information brochure of CSAB 2021 is not  updated in the official website yet.

So the 2020 rules states that

• All the candidates who have accepted and paid the Partial Admission Fee against the JoSAA 2020 (https://www.collegepravesh.com/admissions/josaa/) allotted seat, and want to participate in the CSAB-2020 special Round are required to register on the CSAB portal and pay the CSAB special round non-refundable processing fee.
• Their partial admission fee already paid will be considered as Participation Fee for CSAB special rounds and this amount will be transferred to the finally admitting Institution.
• Such candidates will be participating in special rounds while retaining their seat for which they have already secured admission after JoSAA rounds. But, in case a new seat is allotted in the CSAB-2020 special round, then the previously allotted seat of JoSAA rounds will be automatically cancelled and the candidate will not have any claim on the previously secured seat.
• The candidates who had already paid the seat acceptance fee during the JoSAA 2020 (https://www.collegepravesh.com/admissions/josaa/) rounds but their seat was withdrawn/ cancelled (Last Allotted Seat which was Withdrawn/Cancelled must be from NIT+ system only) after Document verification are required to pay the CSAB special round non-refundable processing fee.
• Their seat acceptance fee already paid will be adjusted against the Participation Fee for CSAB special rounds and this amount will be transferred to the finally admitting Institution. This will not be applicable if the last allotted seat is withdrawn/cancelled from IITs.

Hope I am clear here!

let me know if you want to know anything else.