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SNOMED CT is designed to allow the International Edition to be enhanced by adding Extensions that address national or local requirements. Additional content required to support national, local or organizational needs that may not have international relevance or may not meet the editorial guideline for inclusion in the International Edition.
SNOMED CT design also includes the Reference Set mechanism which provides a standard way to customize and enhance content for use in a particular  languageparticular language, specialty, application or context. Reference Sets developed nationally or locally can modify search and display of content from the International Edition as well as enhancing Extension content.

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The fact that an organization is permitted to produce Extensions does not imply a seal of approval of the quality of Extensions that organization produces. Therefore a person or organization that authorizes or installs an Extension does so entirely at their own risk.

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Reference Sets

SNOMED CT has a broad clinical coverage and includes a depth of detail appropriate to a range of health care disciplines and clinical specialties. As a result, it has extensive content, different parts of which are needed in particular environments. The SNOMED CT design includes the Reference Set mechanism, which provides a standard way to refer to a set of SNOMED CT components and to add customized information to a component.
Organizations implementing SNOMED CT benefit from Reference Sets because they allow requirements for use of particular descriptions and concepts to be represented in a standard form that can be applied to any SNOMED CT enabled application. This allows Reference Sets to be shared throughout and between organizations, even when different software is used to meet local or departmental requirements.
Software developers and vendors benefit because Reference Sets provide a common, machine processable representation of requirements for different patterns of use of SNOMED CT. This simplifies local configuration and enhances interoperability with other SNOMED CT enabled applications.
Reference sets can be used for many different purposes, including:

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