"A word, letter, number, or other symbol used in a code system to mark, represent, or identify something." - Dictionary.com. This data type represents a "coded element" - some label made up of a series of letters or numbers which can be referenced in a code system or value set. A code must be unique within a given coding system, but may not be unique across coding systems. For example, "AU" might mean "Gold" in a chemistry coding system, while it might mean "Australia" in a country coding system. This is why this coded element class contains both the code and the coding system in which it is defined (one can think of a coding system as a namespace in some respects). Each target standard (which we are treating as "platforms" in a model-driven architecture sense) represents coded concepts differently. Some standards, like HL7 v2 have multiple "flavors" of coded concepts. When generating artifacts based on a particular standard, this data type will be substituted with a specific structure particular to the target standard. Note that HL7 FHIR contains an additional element, "primary", which is not in the FHIM. Primary is used to signify which code in a collection of codes is the one that was chosen by the user (as opposed to translations of that code). "A Coding is a representation of a defined concept using a symbol from a defined code system" HL7 FHIR, Coding