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Reference Sets can be distributed as part of the International Release, as part of a National Edition or as part of an Affiliate Edition, and the way the reference set is distributed will typically depend on how the Edition is distributed. However, in some situations, reference sets may be distributed independently. 

Distributing reference sets usually involves performing any additional quality checks required to ensure that the data can be exported correctly, and then exporting the reference set from the terminology management tool. Finally, the reference set is distributed to the users, for example as release files accessed from an online library using a terminology server API. 

Distribution Format

The standard format for distributing SNOMED CT reference sets are the reference set format, as described in  4.2. Common Reference Set Format.  For example, an extensional definition of a subsets of components is represented as a simple reference set, and the standard format for distributing intensional definitions of concept subsets is as a query specification reference set. As part of the SNOMED CT release format, reference sets support unique identification, versioning and recognition of dependencies. 

Using the reference set format for distributing SNOMED CT derivatives is beneficial from the view of maintenance, because the versioning attributes is identical to the versioning attributes used for SNOMED CT components. This means that the checks that need to be done as part of a regular release cycle is easier to manage compared to having a range of different versioning mechanisms to adjust to. 

Other distribution formats may be used where necessary. For example to comply with requirements for representation of value sets including codes from other code systems. However, care should be taken to ensure that distributed subsets are uniquely identified and versioned. Subsets need to be accessed, selected and where appropriate bound to information models using standard approaches.

Release Cycle

Some subsets will be distributed biannually as part of the International Release. However, some others may not require this release schedule. Release processes must include a period of time dedicated to subset manipulation, following the 'freezing' of  SNOMED CT itself. This is the period where all the changes has been made to the core components of SNOMED CT. Timing of release and distribution of specific subsets will be dictated by individual use case-specific requirements. These should be identified and documented initially as part of the subset development process. These may change over time based on user feedback.

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