The appearance of presumptions and fictions in law is associated with such property as formal certainty. For tax law, formal certainty is of particular importance. This is due to the fact that tax law is a public branch of law, which is characterized by complexity and dynamism, regulates highly politicized and conflicting relations regarding the financial support of public entities. Presumptions and fictions as a means of legal technique help to achieve certainty in tax law in cases where this cannot be achieved by other means of legal regulation.