The field behind BR-DE-15
The buyer reference is BT-10. It helps the recipient allocate an invoice to the correct internal process. For XRechnung it is required. It is not the same as the buyer’s electronic routing address.
Ask for the real reference
Public-sector recipients may require their Leitweg-ID in this field. Other recipients may provide a purchase-order or internal reference. Ask what they require; do not paste an example Leitweg-ID or guess one from a company number.
Verify the corrected original
Have the issuer add the agreed reference, export a new original and inspect it again. Passing BR-DE-15 checks presence; it cannot confirm that the reference belongs to the intended recipient. See the rule explanation.