It’ll happen in Delhi ,they’ll call by the ranks to stay in a queue and ask you to choose the branch of AIIMS you want,till all the seats are occupied in the first round (irrespective of the category )
Later ,obc students will be called again ,and the the seats allotted for the obc category will be assigned.
Suppose you took a seat in the general category and you belong to the obc ,you can change it to the branch you want based on the availability of seats .