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Coding references a symbol from a defined code system.   It uses a code defined in a terminology system to associate semantics with a given term, codelist, variable, or group of variables. The presence of a Coding element associates a meaning to its parent element. Including multiple Coding elements for a given parent indicates synonymous meanings provided by different code systems or code system versions.

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The Code is a string pattern that identifies a concept as defined by the code system.

The System is a URI that identifies the code system that defines the code. If the code is taken from a code system resource then the URL for the code system should be used to represent the System. SystemName is a readable name for the System. SystemVersion identifies the version of the System. 

The href is a URI that references a definition of the code within the code system.

CommentOID references a unique ID of a CommentDef element that provides the rationale for the use of the Concept.