If you have already entered marks you will need to make a choice about Rescaling existing grades:
If set to Yes, the previously entered marks will be rescaled so that the percentage remains the same. E.g. If the mark is 20/100 (20%), changing the maximum grade from 100 to 30 for a will rescale the mark to 6/30 so that the mark is still 20%.
If set to No, the previously entered marks will not be rescaled. E.g. For the mark of 20/100 (20%), the mark will stay the same and will now be 20/30 (66.67%).