JOSAA stands for Joint seat allocation authority whereas csab stands for central seat allocation board. While JOSAA is concerned with the seat allocation of the first seven rounds of engineering colleges. CSAB performs the seat allocation of the special rounds and SFTI and NEUT counselling. You can register in both CSAB and JOSAA. Hope this helps.

Dear aspirant,
CSAB and JOSAA are different. JOSAA conducting seven rounds for allotting engineering seats in IITs, NITs, IIITs, GFTIs for different courses like Cse, Ece, etc. After completing seven rounds, CSAB conducting two special rounds for the remaining seats in that 7 rounds. It is okay to register in both of them. You can also register in both. Hope it helps for you.
Thank you