> For the complete documentation index, see [llms.txt](https://docs.snomed.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.snomed.org/snomed-ct-specifications/snomed-ct-release-file-specification/appendices/appendix-b.-specification-reference-information/i/identifierschemeid-field.md).

# identifierSchemeId (field)

A field in the RF2 [Identifier file ](/snomed-ct-specifications/snomed-ct-release-file-specification/appendices/appendix-b.-specification-reference-information/i/identifier-file.md)containing a SNOMED CT identifier which identifies the alternate code system.

Note: In practice, the identifier file is not used in the SNOMED CT International Release as the use of [Simple Map from SNOMED CT Reference Set ](/snomed-ct-specifications/snomed-ct-release-file-specification/reference-set-release-file-specification/5.2-reference-set-types/5.2.3-map-reference-sets/5.2.3.1-simple-map-from-snomed-ct-reference-set.md)or [Simple Map to SNOMED CT Reference Set](/snomed-ct-specifications/snomed-ct-release-file-specification/reference-set-release-file-specification/5.2-reference-set-types/5.2.3-map-reference-sets/5.2.3.2-simple-map-to-snomed-ct-reference-set.md) is preferred. The only current use of this file is for internal identification during the development process.

## Related Links

* [Identifier](/snomed-ct-specifications/snomed-ct-release-file-specification/appendices/appendix-b.-specification-reference-information/i/identifier-file.md)

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