BR-AA-10
A VATBReakdown (BG-23) with VAT Category code (BT-118) "Lower rate" shall not have a VAT exemption reason code (BT-121) or VAT exemption reason text (BT-120).
What the rule checks
As the rule set's publisher wrote it. The rule runs on every element the context matches, and fails where the test is false; the validator reports the line of that element.
cac:TaxTotal/cac:TaxSubtotal/cac:TaxCategory[(normalize-space(cbc:ID) = 'AA' or normalize-space(cbc:ID) = 'RED' or normalize-space(cbc:ID) = 'INT')]
not(cbc:TaxExemptionReason) and not(cbc:TaxExemptionReasonCode)
Checked with Portugal CIUS-PT Invoice (UBL), Portugal CIUS-PT Credit Note (UBL)