By default, the pass mark for unit and level exams is 65%. You can change the pass mark per class via Teach > Course settings.
Note: Make sure all other settings stay identical!
If a student fails an exam, they will be able to take a resit after completing a number of remedial exercises. If a student fails the resit, they will be placed at the beginning of the unit. If you do not want them to redo the unit, you can manually move them to the next unit.