For an intra-community supply (category K) EN 16931 requires the Seller VAT identifier (BT-31) and the Buyer VAT identifier (BT-48) — both are preconditions for the exemption.
Provide both parties' VAT ids (BT-31 and BT-48).
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Rule source: EN 16931 / XRechnung (KOSIT) or Billhorse check (BH-*).