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As explained in 4.2 Modules, every extension must contain at least one module, and all content in an extension must belong to one of its extension modules. More than one module can be created within the same extension, if there is a business requirement to maintain or publish sets of components separately. The set of module concepts that are maintained by the same extension producer are grouped together in a single module subhierarchy.

Caption reference
CapRefId screenshot-of-the-module-hierarchy-from-uk-edition-20170401 CapRefType Figure

 below shows the module subhierarchy for the UK's National Release Center from the 20170401 UK Edition.

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CapId screenshot-of-the-module-hierarchy-from-uk-edition-20170401 CapType Figure
Module hierarchy from 20170401 UK Edition

Please note that some module concepts in the subhierarchy (e.g.

Concept
ShowParts term t |United Kingdom maintained clinical module (core metadata concept)|

)serve as grouper concepts, which are never actually used as the moduleId of any content. The use of separate modules in the UK extension allow them to maintain and publish separate editions that include different content (e.g. with and without the drug extension). Using separate modules also allows module-based filters to be applied when searching content for a specific use case. 

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