The currency

The currency code sets the currency of every amount on the invoice, the amount due included. It must appear exactly once in the settlement block: if it is missing, a business rule reports it, and if it is written twice, the duplicate is what has to go, even when both carry the same value.

The subtlety comes from VAT, because it is the one place where an invoice may legitimately speak two currencies. The total VAT amount may only be expressed in the invoice's currency or in the VAT accounting currency, the one the tax is declared in. Any other currency is refused.

And the attribute counts as much as the value. A VAT amount whose currency attribute is absent triggers the same finding as a foreign currency. The check does not guess that the amount is "probably" in the invoice's currency: an amount that does not state its currency is not in a permitted one, it is in none.

Once per currency, no more. The VAT total may appear only once in the invoice's currency. A second amount is admitted there only if it is expressed in the accounting currency. And if that second currency is declared, only one amount may carry it: two amounts for the same currency are a duplication, not two pieces of information.

In practice, nearly all of these errors reach perfectly ordinary euro invoices, issued by a tool that writes the amount without its attribute. The invoice is right, the currency is obvious to everyone, and the file is refused over a piece of information nobody thought to write because it went without saying.

Open your invoice: the report says which amount is the problem and why, with the rule's official text. Nothing leaves your device. For the other shape checks, see structure errors.