A core responsibility of an NRC is to make the International Edition of SNOMED CT and/or the National Extension of SNOMED CT available for Affiliates.
Every NRC must therefore be able to provide guidance on access to SNOMED CT, version control and management. Moreover, the NRC should be able to provide guidance to users on specific terminology services that are provided by the NRC, and in the use of national SNOMED CT developments, such as National Extensions, Reference sets, etc.
IHTSDO maintains and delivers shared content and derivatives ¹ that provide the foundation of SNOMED CT. This is known as the SNOMED CT International Edition.
SNOMED CT is distributed as sets of files containing terminology components and derivatives. These contain the basic data needed to implement SNOMED CT in various modes in a technology-neutral formats. The format, content and names of the files delivered conform to SNOMED CT specifications and guidelines published in Section 5 of the Technical Implementation Guide (http://snomed.org/tig ).
Components represent the content of the terminology.
The standard SNOMED CT representation for content is as three interrelated files. The Concept file contains unique identifiers for clinical ideas, the Description file links human readable terms with identified concepts, and the Relationship file represents associations between identified concepts.
Derivatives facilitate the effective use of the terminology.
The standard SNOMED CT representation for derivatives is a consistent but flexible file format, known as the Reference set format. Reference sets can be used for a wide range of purposes including subsets, language preferences, ordered lists, hierarchies, annotations and mappings to or from other terminologies, classifications and code systems.

Access to SNOMED CT release files:

Members of IHTSDO and Affiliates outside of Member countries, both access the release files via IHTSDO's SNOMED CT Licensing and Distribution Services (MLDS).
All files distributed to Members are available from the "SNOMED CT International Edition" product page on MLDS, and can be accessed here: https://mlds.ihtsdotools.org/#/ihtsdoReleases/ihtsdoRelease/167
Access and information about MLDS can be found here: https://mlds.ihtsdotools.org.
IHTSDO has provided a guide on how to use SNOMED CT release files, and that guide is useful to gain knowledge on the content of the SNOMED CT release files and an understanding of how the data in the release files support the many uses of SNOMED CT.
Guidance on how to use SNOMED CT release files can be found in the Technical Implementation Guide http://snomed.org/tig.

Release Lifecycle

IHTSDO provides regular updates to the SNOMED CT International Edition. SNOMED CT is released biannually on the last day of January and July each year. Although changes to this schedule are possible in the future, this release cycle has been maintained regularly since 2002. Similarly, Members and Affiliates that maintain Extensions should regularly release updated versions of their Extensions.
Every release cycle of SNOMED CT follows a certain pattern, which is important for NRCs to be acquainted with, as this cycle reflects periods when an NRC must be prepared to conduct evaluations of the International Edition and reconcile the new release with existing national developments.

Figure 9 A National Edition of SNOMED CT must be aligned with the International Release of SNOMED CT and include time for reconciliation.

Beta Release

Approximately two months before each new public release of the SNOMED CT International Edition, a Beta Release is made available to IHTSDO Members (NRCs). Although IHTSDO completes a thorough quality assurance (QA) process on each International Release, the Beta Release provides an opportunity for key stakeholders to perform independent QA on the files and provide feedback to IHTSDO prior to distribution of the final release. NRCs are expected to provide a written report on any issues encountered, to which IHTSDO will formally respond.
The Beta Release is only for QA use by Members. It should not be distributed to Affiliate Licensees or any other third parties without the permission of IHTSDO.
Examples of technical quality assurance tests applied to SNOMED CT releases:

• Standard technical QA - SQL database queries
• Testing the technical integrity of files
• Load testing – loading data into a schema
• Assure integrity of release files
• Pick up integrity flaws with the files
• Acceptance test log – ability to audit the release process

While IHTSDO undertakes these tests, an NRC may choose to replicate them or extend some of these tests in their own technical environments to provide additional quality assurance.

Members' Release

Regular changes to SNOMED CT are necessary in order to satisfy user needs and requirements and to improve the quality of SNOMED CT itself. An NRC must be aware that any change to SNOMED CT may have impact on National Extensions and guidance. Therefore, every release requires reconciliation to ensure that all National Extensions remain aligned with the International Edition. To accommodate this need for reconciliation, Members gain access to each International Release one month before the public release of the SNOMED CT International Edition. The purpose of this Members' release is to allow the NRCs to reconcile their national developments according to the new International Edition prior to the public release.
If SNOMED CT components (concepts, descriptions or relationships) are added or inactivated, an assessment of how these changes affect a National Extension is be required, and adjustments to the national developments may be needed. For example:

• Added concepts may require translation and may also be candidates for addition to national Reference sets.
• Inactivated concepts that members of a National Reference set may need to be replaced with an appropriate active concept.

Release files

With every SNOMED CT release three different release file types are made available:

• Full Release: containing the complete history of every component
• Snapshot Release: containing the current state of every component
• Delta Release: containing only the additions and changes since the previous release

The International Edition is provided in all three release types. However, as the Snapshot and the Delta can be generated from the Full Release, the specification only requires that organizations that maintain Extensions provide a Full Release of their extension.
Guidance on import of the different release types can be found in the following section of Technical Implementation Guide: http://snomed.org/tig.