Hello,

There is no provision to hold the seat secured in JoSAA counselling and participate in CSAB counselling. Candidates who haven't cancelled the seat allotted in CSAB counselling or who haven't withdrawn the seat allotted in JoSAA counselling are not eligible to participate in CSAB counselling. The eligibility criteria to participate in CSAB is that

• A candidate should have a valid JEE Main rank
• A Candidates who participate in JoSAA 2021 but didn't get seat allotment is eligible for CSAB 2021 special round.
• A candidates who opted not to participate in the special round after JoSAA will not be eligible for CSAB 2021 special round admission procedure.

• A candidate who is allotted seats through JoSAA 2021 but haven't accepted their admission or cancelled the seat will also be eligible for CSAB 2021 special round.

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So to participate in CSAB round you need to give your willingness and cancel the seat allotted in JoSAA counselling, if any. You can then participate in CSAB counselling. If you get seat in CSAB counselling you may accept the seat but if you didn't get seat you cannot claim the seat cancelled in JoSAA counselling. Here is the link where you can find the eligibility criteria for CSAB counselling

https://engineering.careers360.com/articles/csab