[The coded attribute allows local codes or user entered text to be sent when the concept that the user would like to express is not a member of the defined value set. If the concept that the user wants to express is represented by a code in the standard value set, the standard code must be sent. The local code can be sent in addition to the standard code if desired. Only if the concept that the user wants to express is NOT represented by a standard code, can a solitary local code be sent. ( HL7V3.0 )]
UMLS (HL7) C1552868 Relation/PAR: Extensibility