Two fields, two decisions
Codice Fiscale uses the TAX_CREDENTIAL_IT localized field; PEC uses TAX_EMAIL_IT. CF Ready does not invent alternative names or repurpose Address 2.
Both fields can be unmanaged, optional and validated, or required and validated. PEC can also become required only when the customer fills in Company in the billing address; when Company is empty, PEC stays optional and is validated when present.
When rules apply
Shopify decides which fields to show. Rules apply to deliveries in Italy. They do not apply if the billing address is outside Italy. If the delivery country is missing, required fields that Shopify does not show do not block the order.
One field with a recognizable label reduces errors and keeps the data where Shopify expects it.
What an app can check
Codice Fiscale checks can cover structure, date, municipal code, omocodia and final check character. PEC can be checked for email-address form. Neither establishes the holder’s identity or proves that a mailbox is an active certified PEC.
Why the theme is not enough
A theme field belongs to the storefront, while checkout is managed by Shopify. It also risks producing a separate value from the native tax field. Localized fields avoid that split.
Continue with how to require Codice Fiscale or why not to use Address 2.