Each VAT breakdown (BG-23) shall have a VAT category rate (BT-119), except if the Invoice is not subject to VAT.
cac:TaxTotal/cac:TaxSubtotal
exists(cac:TaxCategory[cac:TaxScheme/normalize-space(upper-case(cbc:ID))='VAT']/cbc:Percent) or (cac:TaxCategory[cac:TaxScheme/normalize-space(upper-case(cbc:ID))='VAT']/normalize-space(cbc:ID)='O')