# Reference Set

A **reference set** is defined as: a standard format for maintaining and distributing a set of references to SNOMED CT components.

#### Notes

* A *reference set* can be used to represent a subset of components (concepts, descriptions or relationships).
* A *reference set* may also associate referenced components with additional information such as:
  * Ordered lists of components
  * Sets of associations between components
  * Mapping between SNOMED CT concepts and other systems codes, classifications, or knowledge resources.

Hence, reference sets are a mechanism that can be used to represent subsets and value sets of SNOMED CT components. However, they can also be used for many other purposes, including those summarized in the [Use Cases](/snomed-ct-practical-guides/snomed-ct-reference-set-guide/3-requirements-and-use-cases/3.2-use-cases.md) section. The following table lists the attributes used in the standard reference set format, and their purpose.

Table: Standard reference set format

<table><thead><tr><th width="244.265625">Attribute</th><th>Purpose</th></tr></thead><tbody><tr><td><em>id</em></td><td><mark style="color:blue;"><strong>General versioning information</strong></mark></td></tr><tr><td><em>effectiveTime</em></td><td><mark style="color:blue;"><strong>General versioning information</strong></mark></td></tr><tr><td><em>active</em></td><td><mark style="color:blue;"><strong>General versioning information</strong></mark></td></tr><tr><td><em>moduleId</em></td><td><mark style="color:blue;"><strong>General versioning information</strong></mark></td></tr><tr><td><em>refsetId</em></td><td><mark style="color:blue;"><strong>Reference set identifier</strong></mark></td></tr><tr><td><em>referencedComponentId</em></td><td><mark style="color:blue;"><strong>Component reference</strong></mark></td></tr><tr><td><em>&#x3C; attribute-1 _ … attribute-n ></em></td><td><mark style="color:blue;"><strong>Additional information depending on reference set type</strong></mark></td></tr></tbody></table>

## Reference Sets in an Extension

SNOMED CT is designed to allow the International Edition to be extended to meet national or local requirements. A SNOMED CT Extension may contain components of various types including concepts, descriptions, relationships and derivatives including reference sets. Since the International Release and extensions share a common structure, the same application software can be used to enter, store and process information from different extensions. Extensions can also be shared without requiring additional software procurement or development. Most Extensions are managed by Members or Affiliates of SNOMED International.

Reference sets authored in an extension use the same common format as those in the International Edition. This makes it easier to use the same software to create, maintain and share extension reference sets, as used by international reference sets. The reference set format can also be customized in an extension using a standardized customization approach, as explained in section [Pre-defined and Customized Reference Sets](/snomed-ct-practical-guides/snomed-ct-reference-set-guide/4-reference-set-design.md#pre-defined-and-customized-reference-sets).

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