Under art. 24-bis of the Italian income tax code (TUIR, renumbered from 2027), Italian income tax on foreign-source income is replaced by a fixed annual substitute tax, whatever the amount of that income. Income produced in Italy is taxed under the ordinary rules.
The flat tax cannot be combined with the researchers' regime or with the impatriate regime for moves up to 2023, and for people who move from tax year 2027 it can no longer be combined with the current impatriate regime either (D.L. 38/2026). If you have both an Italian salary and significant foreign income, the order of decisions matters: regime first, date of the move second.