# Expressions in a Terminology Server

The terminology services required for postcoordinated expressions are similar to the ones used when authoring content within a published SNOMED CT edition.

When submitting postcoordinated expressions to a terminology server, the expressions can be seen as a derivative, or a supplement, to the SNOMED CT Edition content available in the terminology server. Postcoordinated content lives alongside and can only be interpreted or understood in the context of a specific SNOMED CT Edition version.

Because of the dependency on a versioned SNOMED CT Edition, the successful implementation of expressions involves services to continuously ensure validity and traceability. Similar to the approach to creating SNOMED CT extensions containing clinical concepts, postcoordinated expressions, therefore, need to follow the general principles for extensions, including validation, classification, and versioning.

As illustrated in the image below, the terminology server should, therefore, be designed to hold the given versioned SNOMED CT Edition, and an expression repository to hold the postcoordinated expressions. Furthermore, in addition to the services required by the EHR to use the content and derivatives of released SNOMED CT Editions (see [Terminology Services Guide](https://docs.snomed.org/snomed-ct-practical-guides/snomed-ct-terminology-services-guide/)), services are required to support the use of the postcoordinated content.

<figure><img src="/files/44btu0WQ3fFckO1TCGox" alt=""><figcaption><p>Overall EHR design with a terminology server supporting postcoordination.</p></figcaption></figure>

The following pages will elaborate on the requirements, overall design, and implementation of a SNOMED CT-enabled expression repository, and present the services needed for supporting expressions in a terminology server.

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