This is only valid for those cantons where the employer is allowed to annualize the wages for the calculation of the withholding tax.
The method of calculation is chosen in the company's parameters in the 13th tab.
The program will take into account that there will be a 13th at the end of the year and apply the annual rate from the beginning of the year, so the 13th will have the same tax rate as the other months.
The program will just do it for GE, VD, VS, FR and TI. If an employee is concerned by another canton, it won't do it.