JEE Mains is the entrance examination for admission to IIITs, NITs, IITs, and CFTIs. Along with these premiere Institutions, JEE Mains is also the entrance examination for admission to State run Government and Private Engineering Colleges. To qualify for JEE Mains, there is no minimum aggregate criteria. But if you want to join NITs, IIITs or CFTIs then you should score a minimum aggregate of 75% only in your 12th. 11th marks aren't considered. And then to appear for JEE Advanced which is the gateway for admission to IITs, you will need to fulfill the 75% aggregate condition. Now it is 75% for General Category and 65% for Reserved Categories. So how much you score in 12th is what matters to secure your eligibility only  for NITs, IIITs, CFTIs and JEE Advanced. The rest Institutions do not have this 75% aggregate criteria.