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SNOMED CT is designed to allow the International Edition to be enhanced by adding Extensions that meet national or local requirements. Extensions are managed by Members or Affiliates who have been issued with a Namespace Identifier, which distinguishes the Identifiers of the Components they maintain. An Extension may contain Components of various types (e.g. Concepts, Descriptions, Relationships, and Derivatives including Reference Sets used for a variety of purposes).

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SNOMED CT is a detailed clinical terminology which covers a broad scope. However, some groups of users will need additional Concepts, Descriptions or Reference Sets to support national, local or organizational needs.

This section explains the structures that enable Members ( National Release Centers) and SNOMED International Affiliates to add Concepts, Descriptions, Relationships and Reference Sets to complement the SNOMED CT International Release.

The Extension mechanism allows SNOMED CT to be adapted to address the terminology needs of a country or organization which are not met by the International Release. The mechanism provides a structure within which the components of each Extension are uniquely identified and attributed to a specific issuing organization. This ensures that, when instance data containing content from different Extensions if communicated, the provenance of each referenced Concepts is clear and ambiguity is avoided. Since the International Release and all Extensions share the same common structure, the same application software can be used to enter, store and process information from different extensions. Similarly, Reference Sets can be constructed that refer to content from the International Release and a variety of Extensions.

The common structure also means that, content developed by one organization can where relevant be easily submitted for possible inclusion in a National Edition or in the International Edition.

Using the extension structure can also help organizations transfer responsibility for terminology to the SNOMED International or to another organization, subject to the term of the Affiliate License.