What is meant by 'set on and set off' of allocable surplus under the Payment of Bonus Act, 1965?

Answer 6A(i) Section 15 of the Payment of Bonus Act, 1965 deals with Set on and set off of allocable surplus. It provides that: Where for any accounting year, the allocable surplus exceeds the amount of maximum bonus payable to the employees in the establishment under Section 11, then, the excess shall, subject to a limit of twenty per cent of the total salary or wage of the employees employed in the establishment in that accounting year, be carried forward for being set on in the succeeding accounting year and so on up to and inclusive of the fourth accounting year to be utilized for the purpose of payment of bonus in the manner illustrated in the Fourth Schedule. Where for any according year, there is no available surplus or the allocable surplus in respect of that year falls short of the amount of minimum bonus payable to the employees in the establishment under Section 10, and there is no amount or sufficient amount carried forward and set on under sub-section (1) which could be utilized for the purpose of payment of the minimum bonus, then, such minimum amount or the deficiency, as the case may be, shall be carried forward for being set off in the succeeding accounting year and so on up to and inclusive of the fourth accounting year in the manner illustrated in the Fourth Schedule. The principle of set on and set off as illustrated in the Fourth Schedule shall apply to all other cases not covered by sub-section (1) or sub-section (2) for the purpose of payment of bonus under this Act. Where in any accounting year any amount has been carried forward and set on or set off under this Section, then, in calculating bonus for the succeeding accounting year, the amount of set on or set off carried forward from the earliest accounting year shall first be taken into account. Apart from the provisions contained in Section 15(1), there is no statutory obligation on an employer to set apart any part of the profits of the previous year for payment of bonus for subsequent years.