[ibr-134-ae]-Seller VAT Identifier (IBT-031) MUST be there, except when the Invoice type code (IBT-003) is 'Out of scope of VAT' or 'Credit note related to goods or services'.
ibr-134-ae/ubl:Invoice | /cn:CreditNote(not((cbc:InvoiceTypeCode | cbc:CreditNoteTypeCode) = "480" or (cbc:InvoiceTypeCode | cbc:CreditNoteTypeCode) = "81") and cac:AccountingSupplierParty/cac:Party/cac:PartyTaxScheme[cac:TaxScheme/cbc:ID = "VAT"]/cbc:CompanyID) or ((cbc:InvoiceTypeCode | cbc:CreditNoteTypeCode) = "480" or (cbc:InvoiceTypeCode | cbc:CreditNoteTypeCode) = "81")