warningFactur-X / ZUGFeRD containercontainerPDF
The embedded invoice's /AFRelationship must be /Alternative.
The embedded invoice's /AFRelationship is missing or is not /Alternative.
Set /AFRelationship to /Alternative on the embedded file specification. Factur-X says the XML is an alternative representation of the same invoice; any other value tells the receiver the attachment is something else.
Nothing is stored, and the whole invoice is checked, not just this rule. A PDF only works if it is Factur-X / ZUGFeRD — a PDF with the invoice XML inside it.