Identifier Systems

Identifier systems are composed of non-coded strings (typically numeric or alphanumeric) that have a common purpose and/or context. Typically, identifiers are used to connect content in resources to external content available in other frameworks or protocols. The Global Trade Item Number (GTIN) system and US state driver's license number systems, country passport number systems are examples of external Identifier Systems.

The deferral of the association between a coded element and its allowed coded values is useful because it may not be possible to achieve consensus on the Value Set to be used within a model at the level at which the model is being developed. For example, it may be possible to gain international consensus on messages for submitting insurance claims for health care services. However, gaining international consensus on a single set of billing codes is unlikely any time soon.

Concept Domains are universal in nature, so the name for a Concept Domain should never contain any reference to a specific implementation context. Concept Domains are also present to permit binding to more than one specific terminology (in general), so the names also should not contain a reference to a specific coding system.

A Concept Domain should be documented by specifying a name and a narrative definition. It is strongly recommended that at least three concept examples that represent possible values for an attribute or data type property are provided to illustrate the semantic space. [5] The examples should represent concepts that characterize the intent and purpose of the Concept Domain. This can be accomplished in one of the following ways:

• Including example concepts as part of the narrative definition
• Binding the Concept Domain to a Value Set that provides appropriate example concepts

Examples of Concept Domains

The Concept Domain named 'HumanLanguage' carries the description, "Codes for the representation of the names of human languages". The set of concept identifiers that represent different human languages can be drawn from different Code Systems, depending on the implementation context. For example, a United States Implementation Guide may choose to use a Value Set that includes concept identifiers for various Native American languages, while a New Zealand Implementation Guide may find such a Value Set inappropriate.

Sub-Domains

One Concept Domain may be defined to be "sub-domain" of another. This means that the intended meaning and reference of the sub-domain is intended to be narrower than the meaning of the parent. For example, there is a domain ObservationMethod, with a sub-domain of GeneticObservationMethod. This is not intended to be an ontological assertion; its primary purpose is to indicate that all of the concepts in a Value Set that is associated with the sub-domain should also be concepts for the parent domain, though the reverse may not be true.