> 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/reference-set-release-file-specification/5.1.2-extending-the-basic-reference-set-member-file-format.md).
# Extending the Basic Reference Set Member File Format
The basic reference set file structure enables representation of subsets fo SNOMED CT components. However, the reference set format is extensible, allowing it to be used for a wide range of other purposes. For more details about the requirements that reference sets can address please see the [Practical Guide to Reference Sets](https://docs.snomed.org/snomed-ct-practical-guides/snomed-ct-reference-set-guide/).
The basic reference set structure can be extended by adding one or more fields. Each of these fields will hold additional specific values related to each member. Three general data types are supported in the additional columns. These are
* Integer
* String and
* Component (a reference to a SNOMED CT component)
Finer grained interpretation of these data types can also be specified using a special metadata reference set known as the [900000000000456007 | Reference set descriptor|](http://snomed.info/id/900000000000456007) .
The Reference Set patterns that are supported as part of the International Edition are documented in [Reference Set Types](https://github.com/SNOMED-Documents/snomed-release-file-specification/blob/main/5%20reference-set-release-files-specification/5.2%20reference-set-types). Additional reference set patterns can also be created as part of an extension to support additional use case.
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