The tax amount for reverse charge VAT MUST be zero. (since there is only one VAT category allowed it follows that the invoice tax total for reverse charge invoices is zero)
/ubl:Reminder//cac:TaxTotal/cbc:TaxAmount = 0 and (//cac:TaxCategory/cbc:ID) = 'AE' or not((//cac:TaxCategory/cbc:ID) = 'AE' )