Determinant calculation of tax at source 13th salary

Circular 45 on withholding tax on employees (french | german | italian) published by the Swiss Federal Tax Administration stipulates in article 7.1:

If withholding tax is calculated using Swissdec-certified software, the current Swissdec guidelines must be applied for calculating withholding tax. In special situations and/or cases, the withholding tax calculation may deviate slightly from this circular.

By way of derogation from this circular, the 13th salary is taken into account differently in the annual model when calculating using Swissdec-certified software. In this case, to calculate the annual income determining the rate, the 13th salary is taken into account only when the latter is paid, in application of the realization principle. The procedure is defined in the Swissdec guidelines, which have been expressly approved by the tax authorities.