Dear student,

Open category means open to all categories. It is not reserved only for general candidates, it is open to candidates from different categories.
In open category seat, anyone can apply for that seat, either he/she is general or obc or ews or pws o r sc or st.

So, the vacant open category seats are open to all categories, but generally these are acquired by general category candidates. Because for other categories, the seats are specially reserved.

And check out the following link to know more about csab special round counselling and the previous year cutoff Trends ( note that all the mentioned ranks are CRL rank not category wise ranks ) :-
https://engineering.careers360.com/articles/csab-special-round-for-nits-iiits-and-gftis