Hello Student

CSAB means Central seat allocation board which converts the jossa rank  to all India rank for the remaining seats. The seats which are converted jossa rank after the 7th round of counselling the number of seats  are converted into to all India rank where anyone from requisite qualification and eligibility can appear for the vacant seats. This type of conversion is done after the JEE main rank holder candidate is not found to fill up the vacant seats in JoSAA counselling after the 7th round.

Hope this helps.