Hey, here it goes-
You know the CRL(central rank list) is prepaired after the result, the category wise rank lists are also prepaired.
The seats in the colleges are reserved percentage wise. So the seats reserved for one category are filled by their ranklists. As the seats are reserved for the various categories remaining seats which are reserved for the general category which are few make the competition very difficult.
Now comming to the councelling, the councelling to the various IITs and NITs is conducted via JoSAA(joint seat allocation authority). JoSAA prepairs a list of all the seats and courses available at the NITs and the IITs, now after you have qualified your mains and advance you are eligible to take admission in the NITs and the IITs respectively. Now you have to prepair your list of choices. You have to write a code basically that can be read by the seat allocation authority throughout the councelling.
You will be alloted seats rank wise, say you have made a list of choices, now suppose that your rank is 4th, first of all the person with the first rank will get the seat of his choice, then the second person, say the second person has filled the seat that was taken by the first ranker on the top and assuming that there is only one seat of that branch, he will be alloted the seat he filled on the second preference. So you will get the seat which is available and is on the top of your list. For categories it is done on the basis of category ranks.
Now when you have your seat alloted you have to go to any NIT if the seat is alloted in NIT, or in any IIT if the seat is alloted in IIT for document verification after filling their advance payment. Reporting is must for you will not be allowed to participate in further rounds without document verification. There you will have three choices- to freeze your seat i.e. your seat is confirmed, you have taken that seat and no one can interfere with your seat, or you can slide i.e. you seized your seat and now will be alloted a seat of higher preference in the same institute if it becomes vacant in the further rounds, or you can float i.e. you can seize your seat and when any seat of your higher preference will be availabe in the next rounds you will be slloted that seat and your seat will be vacant for othets to take.