KF-PDF-RELATION: The attachment relationship is not "Alternative"
This code shows up when an electronic invoice is validated. Here is what it means, why it fires, and what to change in the file.
Why this code appears
The attachment does not declare /AFRelationship /Alternative. That value says the XML is ANOTHER FORM of the same document rather than an annex: it is what makes the file an electronic invoice instead of a PDF with something stapled to it.
How to fix it
Set the associated file relationship to Alternative when attaching. Data and Supplement are refused for Factur-X.
Where it happens
This check is ours, not a code from the standard: we do not cover enough of EN 16931 to borrow its numbers, and a misattributed official code would send you to fix the wrong field. It concerns PDF.