In EN16931, the order issued by the buyer — known as the "Purchase order reference" (BT-13) — is defined as a mandatory XML element:

BT-13: /Invoice(1..1)/cac:OrderReference(0..1)/cbc:ID(1..1)

By contrast, the "Sales order reference" (BT-14) is defined as an optional XML element:

BT-14: /Invoice(1..1)/cac:OrderReference(0..1)/cbc:SalesOrderID(0..1)

However, in the semantic model of the EU invoice (EN16931-1), both BT-13 and BT-14 are optional.

This discrepancy leads to a practical issue: if only BT-14 is available (and BT-13 is missing), the shared XML parent element (cac:OrderReference) must still be created — forcing the generation of a BT-13 element with a placeholder value such as "none".

Such cases can occur, for example, when the buyer does not explicitly issue an order but selects goods or services directly via the seller’s web interface.