BR-52
Each Additional supporting document (BG-24) shall contain a Supporting document reference (BT-122).
What the rule checks
As the rule set's publisher wrote it. The rule runs on every element the context matches, and fails where the test is false; the validator reports the line of that element.
cac:AdditionalDocumentReference
normalize-space(cbc:ID) != ''
Checked with EN 16931 Invoice (UBL), EN 16931 Credit Note (UBL), Croatia eRačun Invoice (UBL), Croatia eRačun Credit Note (UBL)
//ram:AdditionalReferencedDocument
normalize-space(ram:IssuerAssignedID) != ''
Checked with EN 16931 Invoice (CII)
cac:AdditionalDocumentReference
(cbc:ID) != ''
Checked with Romania RO e-Factura Invoice (UBL), Romania RO e-Factura Credit Note (UBL)
//ram:AdditionalReferencedDocument
(ram:IssuerAssignedID!='')
Checked with France Factur-X EN 16931, Factur-X / ZUGFeRD EN 16931