How do I validate ATU + USt at checkout for Austrian + EU B2B?

Two Austrian B2B identifiers, two validators:

• ATU (Austrian VAT-ID, format ATU + 8 digits, e.g. ATU12345678), validated against the VIES VAT service (EU-wide) for cross-border B2B, or against the Austrian Finanzonline for domestic-only.
• UID-Nr. (Umsatzsteuer-Identifikationsnummer), same as ATU, different name. The two are interchangeable.

For Magento B2B we:

1. Add an ATU field to the customer entity (or B2B company on Adobe Commerce).
2. Validate ATU against VIES on registration, rejects invalid / closed companies, auto-fills company name + address from VIES response.
3. Apply 0% USt on B2B intra-EU sales (reverse charge / intra-Community supply) with a valid VIES-validated ATU; 20% USt on B2C + AT→AT B2B; 0% USt on exports outside the EU.
4. Cache VIES validations 30 days, re-validate on any address change. VIES outages are common, we degrade gracefully to manual-approval queue.
5. Per Austrian Finanzamt rules, store the VIES validation timestamp + response on the order for audit purposes (7-year retention).