Dear student,

CSAB - Central seat allocation board is responsible for selling the vacant seats in NITs or gftis, etc.

Now CSAB is generally of two round counseling and  is conducted for North-Eastern states and Union Territories - CSAB (NEUT), for supernumerary seats in the Union Territories - CSAB (UT) and for Self-financed technical institutes - CSAB (SFTI).
For all these counseling the dates are not yet come.

Now, if you have not got allotted any seat in josaa counselling for you have withdrawn your seat from josaa counselling then you are eligible to participate in CSAB 2020 counselling.

Now in CSAB counselling again you have to a fresh registration and your josaa registration or choice filling doesn't matter here.
So, in csab counselling, you once have to fill the new choices and have to pay the participation fees. After the choice locking once again the counseling for getting the vacant seats will start.

Also click on the following link to know more about csab counseling :-
https://engineering.careers360.com/articles/csab/amp