Dear,
These are terms used in the Bonus Act to decide the percentage of bonus to be paid to employees.
Set on - First, we calculate allocable surplus, and out of that surplus, we pay bonus. After paying the maximum bonus, i.e., 20%, if some amount is left, the balance is called set on, and this amount is kept in reserve for the future.
Set off - If in any accounting year you do not have sufficient surplus and are unable to pay the minimum bonus of 8.33%, then, legally, you have to pay 8.33%. The balance amount that you do not have in surplus will be shown as set off for the future.
You have to take care of set off/on in the future before deciding the rate of the bonus.
Thanks,
J. S. Malik