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There are two distinct ways to define the membership of a subset. These are known asextensional and intensional subset definitions. These terms are defined and illustrated below.

Extensional subset definition

A subset in which the members are represented by enumeration.

Figure 2.1.1-1: Illustration of an extensionally defined subset

Intensional subset definition

A subset definition in which the membership is represented by a set of rules specifying the conditions for inclusion.





Figure 2.1.1-2: Illustration of an intensionally defined subset

Extensional Definitions

Extensionally defined subsets of SNOMED CT components consist of a list of SNOMED CT component identifiers, which are typically either concept or description identifiers. Extensionally defined SNOMED CT subsets are formally represented using the  simple reference settype.

Figure 2.1.1-3: Illustration of an extensionally defined subset of SNOMED CT concepts

Intensional Definitions

The design of SNOMED CT lends itself to intensional subset definitions, because rules can be formulated to specify the conditions for member inclusion.  The subtype hierarchy and formal definitions of SNOMED CT  concepts enable constraints to be specified, such as:

Intensionally defined SNOMED CT subsets are formally represented using a  query specification reference set. The query string in this reference set represents the intensional definition of the subset. The standard way of representing the query is using the SNOMED CT  Expression Constraint Language (ECL).


The substrate is the superset of members to which an intensional subset definition is applied. Related to subsets of SNOMED CT components the 'Substrate' is the SNOMED CT content against which an intensional query is executed. Typically, the substrate is a specified version of a particular SNOMED CT Edition. Regardless of whether the subset members are intentionally defined or extensionally defined it is important to be specific about the substrate used to generate the subset, because the result of running a query, or manually selecting the subset members, may vary depending on the substrate used.


The expansion is the result of applying an intensional definition to a given substrate. The Expansion may differ depending on the substrate that the query is run against. The expansion that results from running the query against a specific SNOMED CT substrate can be represented extensionally as a simple reference set. Alternatively, the expansion may be computed dynamically or represented in an internal format within a software application.

For more information about the process and methods for developing subsets and reference sets, see the section about  refset development


Figure 2.1.1-4: Illustration of a substrate, intentional subset definition and expansion

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