In DNB registration.....what is OBC (A) , OBC (B)?

In DNB registration…what is OBC (A) , OBC (B) ?

Some states have divided OBC in further category according to caste. ( I think Karnataka ,TN , Kerala ). So if you are a general OBC you can’t get OBC A or OBC B or OBC C etc. This is followed in DNB counseling. State students generally know about this