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When implementing SNOMED CT in a clinical software system, such as an EHR, the use of SNOMED CT will typically involve customization. For example, selecting a subset of SNOMED CT components to be used for a particular purpose is a typical way of customizing SNOMED CT for use. Subsets of SNOMED CT components can be used to constrain the use of SNOMED CT by either including, excluding or prioritizing specific components. An extensionally defined subset of components, whether it is a subset of concepts, descriptions or relationships, can be represented as a  simple reference set. The definition of an intensionally defined subset of components can be represented using a query specification reference set , while its expansion can be represented using a  simple reference setA subset of components may be required to support a range of different uses, as illustrated in the table below. 


RequirementDescriptionExample usesReference Set
Subset of concepts

An extensionally defined set of references to SNOMED CT concepts

  • Restricting searches to terms associated with specified concepts
  • Constraining data entry
  • Specifying value sets for particular data items
  • Specifying queries for data retrieval

Simple reference set


An intensionally defined set of references to SNOMED CT conceptsQuery specification reference set  

Subset of descriptions

A set of references to SNOMED CT descriptions
  • Restricting searches to specified sets of terms
  • Specifying descriptions to appear in a list of options
Simple reference set 
Inclusion/Exclusion of content
A set which contains the components to be included/ excluded
  • Excluding particular components from search and/or data entry
  • Including a subset of concepts/descriptions for search, data entry, reporting etc.

For more detailed use case examples, please refer to the following sections: