The official Italian form for tax withholding certificates (for employees, freelancers, pensions) is the "Certificazione Unica" (CU) .
Ministero dell’Economia e delle Finanze – Dipartimento del Tesoro. Account Number: 67422808. bollettino td 674 pdf
Here is an article summarizing the content and significance of this technical document. The official Italian form for tax withholding certificates