Dear Saurav, let's say you appeared for NEET and you qualified too but you haven't appeared for your 12th. You will not be given a seat in the counselling. The reason being you dont have the required academic qualification. 12th is the eligibility criteria for joining Medical, Dental and Ayush colleges. If you dont appear for the Board exams, you will be considered ineligible. NTA doesn't hold on to your seat until you complete your 12th.