Supplementary exams are not compulsory for anyone at IIIT Vadodara. Only if you fail in a particular subject or if you miss the end semester exam of a subject, then you have the option of taking it.
This is a new method of passing a subject which has been introduced by authorities in which you can give the supplementary exam in the subject which would change only your end semester exam marks. Remember that you will still need to score the overall minimum passing marks in the subject. If your combined total of marks even after the supplementary exam is lower than that, you would have to repeat the subject next year.