Practically speaking, a SNOMED CT extension extends the capabilities or scope of the International Edition by providing additional content for use in a particular region or setting. Technically speaking, an extension is a release package containing one (or more) modules which are created and maintained by a single organization. Unlike, the International Edition an extension is not a standalone terminology. Extension modules depend on and must be used with the International Edition modules, and some extensions may also depend on modules from other extensions. This also explains why organizations might be reluctant to provide an extension package on its own and instead provide only an edition. Perhaps the most common type of extension would be one created for national use in a Member country. Extensions are technically consistent with the logical design of the International Edition.

Extension Modules

Every extension must contain at least one module. If an organization has a business requirement to maintain separate modules, they may do so but a minimum of one is required. In these cases the module concepts are organized in the module sub-hierarchy using | Is a|  relationships, as shown in below.

Note that in these cases some module concepts may be created simply as grouper concepts, and content need not be assigned to these modules using the moduleId attribute. Using separate modules allows the UK to publish separate editions with and without the drug extension. Another benefit of defining separate modules is that browsers can be configured to filter by module, which may be useful where content in a particular module is only of interest to specific users or in specific situations.

Versions

Extensions are versioned in much the same way as editions are versioned with the specific version indicated by the date of publication. For more information on this topic, please refer to Versioned Editions. In some cases, a need will exist to refer to a version of an extension, for example when a versioned National Edition indicates which versioned extension(s) are included.