If the tax percentage in a tax category is 0% then an exemption reason SHOULD be provided except in reverse charge VAT.
//cac:TaxSubtotal
(number(cac:TaxCategory/cbc:Percent) = 0 and (cac:TaxCategory/cbc:TaxExemptionReason or cac:TaxCategory/cbc:TaxExemptionReasonCode)) or (number(cac:TaxCategory/cbc:Percent) !=0)