> 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-practical-guides/snomed-ct-postcoordination-guide/expressions-in-a-terminology-server/design/expression-repository-design.md).

# Expression Repository Design

The image in this page illustrates an expression repository contains one or more expression versions.Each expression version *must* contain the following data:

* *Versioning data* , to represent the time and substrate of the given version of the expression
* A *Close-to-user form expression* , to represent the expression as it was stated upon creation

Each expression version *should* contain the following data:

* An *expression identifier* , **if** this is required to uniquely identify the expression
* A *Classifiable form expression* , to enable the generation of the NNF expression
* A *Necessary Normal Form expression* , to support querying the expression

All SNOMED CT concepts referenced by expressions in the repository at a particular point in time should all be included, and active, in a specific versioned edition of SNOMED CT. General expression repository design.

<figure><img src="/files/X0HtrT2SzhhK9pIMxlTC" alt=""><figcaption><p>General expression repository design.</p></figcaption></figure>

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