If talking about Indian Institute of Space Scienec and Technology (IIST), Thiruvananthapuram then JEE Advanced is must as merit lists are prepared on the basis of JEE Advanced Scores and no candidate can apply without entering JEE Advanced Scores so it is necessary.
Talking about Indore Institute of Science and Technology (IIST), Indore then admission procees is done in phases in which first phase is for those who apperaed in JEE Mains exam. Second is for those who could not appear in JEE but have qualifying marks in higher secondary examination and applied for participation in online councelling of DTE. Last phase is for those who have JEE or qualifying marks but could not apply for participation in online councelling process od DTE MP.