BR-KSA-92
If it's a B2G invoice transaction type (KSA 2, position 9 = 01), then the Contract ID (BT-12) must be present in a tax invoice and associated credit notes and debit notes (KSA-2, position 1 and 2 = 01).
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 | /cn:CreditNote
not(exists(cac:ContractDocumentReference/cbc:ID))
Checked with Saudi Arabia ZATCA Tax Invoice (UBL), Saudi Arabia ZATCA Simplified Tax Invoice (UBL)