A structure used to associate a list of code values to specified categories. May be flat or hierarchical. This is a maintainable object. In addition to the standard name, label, and description the CodeList specifies a recommended data type, the hierarchical nature of the CodeList, level descriptions, individual code values and associated category, and whether the CodeList contents are used to represent system specific missing values. May include another CodeList by reference. If including other CodeLists make sure there are no code conflicts in the overall content. A code value must be unique within the CodeList. May also include a reference to a default CategoryScheme.