Value added tax point date (BT-7) and Value added tax point date code (BT-8) are mutually exclusive.
/ubl:Invoice | /cn:CreditNote
(exists(cbc:TaxPointDate) and not(cac:InvoicePeriod/cbc:DescriptionCode)) or (not(cbc:TaxPointDate) and exists(cac:InvoicePeriod/cbc:DescriptionCode)) or (not(cbc:TaxPointDate) and not(cac:InvoicePeriod/cbc:DescriptionCode))