Hello aspirant,

The basic difference is that first, JoSAA completes its JEE Main Counselling process for admissions to IITs, NITs and other participating colleges. Then, CSAB conducts 2 rounds to fill up the vacant seats (if any) of NITs, IIITs and other colleges (this does not include IITs).

https://josaa.nic.in/webinfo/File/GetFile?FileId=24&LangId=P

The link I've provided above contains all the information related to your query.

I request you to kindly go through it properly and in case further any query occurs than do seek us.

Happy to help, hope it helps!