All the back pay, such as salary, overtime, and allowances, will fall under additional wages. Please do as follows to set up additional wages:
Go to the Payroll module and click Pay Element Setup.
Select existing back pay and click on the three-dotted button.
Ensure that Subject to Additional is selected on the right panel.
If not selected then arrow over Subject to Additional from the left to the right panel.
Arrow over Subject to Ordinary from the right to the left panel and Save.