if its a prac, a small tutorial exam or even a mid semester exam they usually dont do supplementary exam for those.
so if you miss the Midsemester worth say 30%, then you will get NIL for that, but your final mark will be out of 70 instead of 100.
generally missing the mid-semester or tutorial exams, is not a good idea, coz they are generally easier. as they cover the early introductory topics where its easy to gain marks.