Hey radhakrishna! 

Open round is for all the seats which left vacant after previous round including surrendered/resigned and cancelled. All the eligible candidates are called for this round. All the student who qualify this exam can be eligible for counselling 

It is offline counselling so you have to go AIIMS delhi on the day of counselling. Generally  50 seats (UR)  are left for open round counselling so if you getting rank under 2500 then you must go for open. 

I think that you have good chances in open round! 

Good luck