BR-KSA-49
If the tax exemption reason code (BT-121) is equal to VATEX-SA-EDU or VATEX-SA-HEA, then the other buyer ID (BT-46) is mandatory and must be national ID (BT-46-1 = NAT).
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[cbc:TaxExemptionReasonCode = 'VATEX-SA-EDU'] | cac:TaxTotal/cac:TaxSubtotal/cac:TaxCategory[cbc:TaxExemptionReasonCode = 'VATEX-SA-HEA']
exists(//cac:AccountingCustomerParty/cac:Party/cac:PartyIdentification/cbc:ID) and (//cac:AccountingCustomerParty/cac:Party/cac:PartyIdentification/cbc:ID/@schemeID) = 'NAT'
Checked with Saudi Arabia ZATCA Tax Invoice (UBL), Saudi Arabia ZATCA Simplified Tax Invoice (UBL)