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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 set. A subset of components may be required to support a range of different uses, as illustrated in the table below. 

Requirements for subsets of components

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

• Constrain data entry
• Constrain searches
• Exclude content
• Categorising patients using subsets
• Constrain value sets