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When a new or updated rule is introduced to the MRCM, it may take a period of time before all existing content conforms to the new rule. In these situations, a content type of |New precoordinated SNOMED CT content only| can be used, to ensure that all new content obeys the rule. Over time, as the old content is updated to reflect the new rule, the content type may be changed to either |All precoordinated SNOMED CT content| or |All SNOMED CT content|. This approach can be used to ensure that all new content added to SNOMED CT conforms to high quality concept model rules, even though reviewing and correcting existing content may take additional time. 

Extension

As described in Requirement D.6, the international MRCM rules defined by the IHTSDO must be able to be extended and adapted by organizations developing SNOMED CT extensions, to support the concept model requirements of their extension content.
To support this requirement, SNOMED CT extension developers will be able to copy the international MRCM rules defined by the IHTSDO into one or more extension MRCM reference sets and edit these reference sets to add new rules and restrict or extend the existing rules as required. New MRCM reference sets created for a SNOMED CT extension must be designed to ensure consistency and data integrity between editions. Please note that if no changes to the SNOMED CT concept model are required to support an extension, no additional MRCM reference sets need to be created, as the international MRCM can be reused by the extension.

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