Bonus and backpay should not be added on one source code. Under which source code was the bonus and arbitration award recorded?
The source code for the Arbitration award is 3608 (taxable) and 3609 (non-taxable). An arbitration award for backpay will be added to your total taxable income for the year.
The source code for bonuses is 3605. Your bonus amount simply becomes part of your total taxable income for the year, which may impact your tax bracket, i.e. the percentage of tax you pay on your income. The amount of tax withheld in your bonus month depends on which method your company uses to calculates tax.