BR-56
Each Seller tax representative party (BG-11) shall have a Seller tax representative VAT identifier (BT-63).
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:TaxRepresentativeParty
exists(cac:PartyTaxScheme[cac:TaxScheme/(normalize-space(upper-case(cbc:ID)) = 'VAT')]/cbc:CompanyID)
Checked with EN 16931 Invoice (UBL), EN 16931 Credit Note (UBL), Croatia eRačun Invoice (UBL), Croatia eRačun Credit Note (UBL)
//ram:SellerTaxRepresentativeTradeParty
normalize-space(ram:SpecifiedTaxRegistration/ram:ID[@schemeID='VA']) != ''
Checked with EN 16931 Invoice (CII)
//ram:SellerTaxRepresentativeTradeParty
(ram:SpecifiedTaxRegistration/ram:ID[@schemeID='VA']!='')
Checked with France Factur-X BASIC WL, Factur-X / ZUGFeRD BASIC WL