BR-KSA-83
A VAT breakdown (BG-23) with VAT Category code (BT-118) 'Exempt from VAT' or 'Zero rated' or 'Not subject to VAT' shall have a VAT exemption (or exception) reason text (BT-120) with one of the values from paragraph 11.2.4 of XML implementation standards on Tax exemption (or exception) reason text - specific to Saudi Arabia.
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.
ubl:Invoice/cac:TaxTotal/cac:TaxSubtotal/cac:TaxCategory[normalize-space(cbc:ID) = 'Z'] | ubl:Invoice/cac:TaxTotal/cac:TaxSubtotal/cac:TaxCategory[normalize-space(cbc:ID) = 'E'] | ubl:Invoice/cac:TaxTotal/cac:TaxSubtotal/cac:TaxCategory[normalize-space(cbc:ID) = 'O']
exists(cbc:TaxExemptionReason)
Checked with Saudi Arabia ZATCA Tax Invoice (UBL), Saudi Arabia ZATCA Simplified Tax Invoice (UBL)