Region or province. Required for every country — the provider rejects a billing address without one, so a create that omits it (or sends a blank string) fails with 422 BILLING_ADDRESS_REGION_REQUIRED and a field error on this field, before the provider is called. For Italy, send the two-letter province sigla. The field stays nullable in this schema for backward compatibility; it is enforced at the boundary, not by schema validation.

Certified email address for electronic invoicing. An Italian BUSINESS or FREELANCE address must carry this or sdiCode; without either, the create fails with 422 BILLING_ADDRESS_ELECTRONIC_INVOICE_REQUIRED. Not used outside Italy, and not required for a PRIVATE address.

Electronic-invoicing recipient code. Interchangeable with pec for an Italian BUSINESS or FREELANCE address: either one satisfies the requirement.