Invoice period description code must be according to UNCL 2005 D.16B.
PEPPOL-EN16931-CL006pxs-taxdata:TaxData/pxs-taxdata:ReportedTransaction/pxs-taxdata:ReportedDocument/cac:InvoicePeriod/cbc:DescriptionCode some $code in $UNCL2005 satisfies normalize-space(text()) = $code