suppose I wrote jee mains and can get a qualified for advanced if I got low marks in advanced as iam a st category student if they give seat or not based on mains score

JEE Mains and JEE Advanced are 2 different examinations. 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 for NITs, IIITs, CFTIs and JEE Advanced. So lets say you did score 65% in your 12th, you qualify for  participating in the Counseling for NITs, IIITs, and CFTIs. Again for being eligible for JEE Advanced, the authorities will release the cut offs and according to your Category, you need to secure that cut off. Lets say now if you secure less than required in JEE Advanced for ST Category, you will not get any exception for admission to IITs. You still can get admission in NITs, CFTIs and IIITs based on JEE Mains score. But you cannot get admission to IITs based on your JEE Mains score.

JEE Mains and JEE Advanced are 2 different examinations. 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 for NITs, IIITs, CFTIs and JEE Advanced. So lets say you did score 65% in your 12th, you qualify for  participating in the Coounseling for NITs, IIITs, and CFTIs. Again for being eligible for JEE Advanced, the authorities will release the cut offs and according to your Category, you need to secure that cut off. Lets say now if you secure less than required in JEE Advanced for ST Category, you will not get any exception for admission to IITs. You still can get admission in NITs, CFTIs and IIITs based on JEE Mains score. But you cannot get admission to IITs based on your JEE Mains score.