Hello.
In the case of self-employed activities, the person who carries out such activities must write to the tax authorities every three months, on a separate form, all the invoices he has issued, separately for legal and natural persons. If they have invoiced natural persons, they must also fill in a separate REK form for each month for the tax administration, on the basis of which the tax administration sends a decision for payment of income tax. I consider that the REK form is unnecessary. The tax administration could issue the income tax decision itself on the basis of the three-month billing statement, as it has the information. In this case, however, not for each month separately, but for the three months together. However, the amounts involved here are small. Or she would not issue any decisions at all during the year, as the total is settled at the end of the year in the income tax decision for the previous year.