Value MUST be part of code list 'Recommendation 20, including Recommendation 21 codes - prefixed with X (UN/ECE)' or 'Recommandation 21 (UN/ECE)'.
/ubl:Invoice/cac:InvoiceLine/cac:Price/cbc:BaseQuantitynot(@unitCode) or (some $code in $clUNECERec20 satisfies $code = @unitCode) or (some $code in $clUNECERec21 satisfies $code = @unitCode)