[ibr-12-ng]-Line VAT information (IBG-30) with Invoiced item VAT category code (IBT-151) as 'Exempt from VAT' MUST have a VAT exemption reason code (IBT-186).
ibr-12-ngcac:InvoiceLine | cac:CreditNoteLinenot(cac:Item/cac:ClassifiedTaxCategory/cbc:ID = "E" and not(exists(cac:Item/cac:ClassifiedTaxCategory/cbc:TaxExemptionReasonCode)))