For wages to be classified as Ordinary Wages (OW) for the month, it must satisfy both conditions below:

The wages are due or granted wholly or exclusively in respect of an employee’s employment during that month; and

The wages for that month are payable* by 14th of the following month.

Wages which are not classified as OW will be Additional Wages (AW) for the month.

Example of OW: monthly salary

Example of AW: annual performance bonus

*Please note that the payable date refers to the date where company is contractually obligated to pay employee the wages. Do note that the Employment Act (EA) also imposes certain requirements on the timing for salary payment if employee is covered under EA. If there are circumstances that resulted in administrative delays in payment e.g. new employee joins after payroll cut-off date, you should use the payable date to determine the classification of wages.

Source: CPF Board - Ordinary Wages (OW) and Additional Wages (AW)