Your secondary tax card only states your withholding rate. When your employer/provider pays you, tax is withheld on the entire amount. By using both your primary and secondary tax cards, you make sure that your deductions and allowances are only applied once. If more than one employer or provider uses your primary tax card, you may end up paying too little tax resulting in outstanding tax the following year.
You only use your secondary tax card if you have more than one employer or provider - if you have a job and receive the state education grant (SU) at the same time, for example.
You may use your secondary tax card for multiple employers or providers.
You need to inform your employer, the state education grant administration or, for example, your pension provider whether to use your primary or secondary tax card.