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The module dependency has specified that 

The OWL ontology reference set should only have one single active entry of of 

A new entry for OWL ontology header should be added with the extension module id and effective time.  The identifier for the ontology URI should be the most dependent module for that particular editiontime for an extension edition of SNOMED CT. The OWL ontology header for SNOMED CT international release should be inactivated.

The OWL ontology header represents the identity of an ontology. It needs to be updated when the OWL expression refeset represents a different ontology, e.g. ontology of a national extension. Note, the changes to the content, version, and module dependency of an ontology do not require to update the ontology header.

It is possible that modules in SNOMED CT can be directly translated into OWL ontologies, with the module dependency reference set describing how these ontologies are imported. Extensions can import the ontology of SNOMED CT core module since the content in the model component module has already been included. The new OWL ontology header should include the extension module identifier and import statement(s). This approach could avoid accidental changes to the imported ontology. However, the implementation could be more complicated than the representation of an entire edition as a single ontology. Therefore, it is not recommended to use an ontology import statement at the current stage. Instead, each edition will be rendered as one OWL ontology, without any ontology import statement.