Value MUST be part of code list 'Recommendation 20, including Recommendation 21 codes - prefixed with X (UN/ECE)'.
/ubl:OrderResponse/cac:OrderLine/cac:SellerSubstitutedLineItem/cac:Item/cac:AdditionalItemProperty/cbc:ValueQuantitynot(@unitCode) or (some $code in $clUNECERec20 satisfies $code = @unitCode)