The Joint Seat Allocation Authority (JOSAA) conducts the admission of candidates into appropriate colleges based on their JEE Main and Advanced scores and ranks. Those who haven’t cleared JEE Advanced but are eligible for the Preparatory course can also select colleges using the JOSAA Portal.If a candidate is eligible for the prep course, then the choices that he/she gets in the JOSAA portal will also include all the IITs. However, it should be noted that these choices are not for the prep course but for the actual courses that the candidate chooses (during the choice filling in JOSAA) and are applicable only after passing the prep course and getting admission into any of the IITs.The center that a candidate gets for the prep course will depend on their Preparatory rank

The IITs have announced the one year preparatory course for SC/ST candidates who have not qualified to be allotted seats for the current year . Students shortlisted for this course must successfully complete it to join the B.Tech courses at the IITs. The Preparatory Courses is a one year residential programmes and as the name suggests prepares the students to be eligible for the B.Tech or Dual Degree Courses at the IITs.

• Every year around 30 to 40 candidates from the non-qualified candidates from the ranking order of JEE are taken.
• Physically Disabled (PD) candidates are also eligible to undergo the preparatory course (open from the year 2009).
• The Joint Admission Committee (JAB) will take the admission decision after candidates successfully complete the preparatory course.
• The IIT Preparatory Course comprises of courses in Mathematics, Physics, Chemistry and English for two semesters after which the candidates are evaluated.
• Candidates who successfully pass in flying colours are thereafter allowed admissions to the vacancies pertaining to SC/ST for the particular year.
• Students may note that the IIT Preparatory course is residential.
• All students who take admissions in this are eligible for all concessions on par with the regular students during admissions to the degree engineering courses

https://josaa.nic.in/webinfocms/public/view.aspx?page=46

Visit this link for more details