[ibr-co-20]-If Self-billed invoice line period (ibg-26) is used, the self-billed invoice line period start date (ibt-134) or the self-billed invoice line period end date (ibt-135) MUST be filled, or both.
ibr-co-20cac:InvoiceLine/cac:InvoicePeriod | cac:CreditNoteLine/cac:InvoicePeriodexists(cbc:StartDate) or exists(cbc:EndDate)