The VAT category tax amount MUST be zero if the VAT category code is reverse charge (since there is only one VAT category allowed it follows that the credit note tax total for reverse charge credit notes is zero)
//cac:TaxTotal[cac:TaxSubtotal/cac:TaxCategory/cbc:ID = 'AE']every $taxamount in cac:TaxSubtotal/cbc:TaxAmount satisfies $taxamount = 0