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A subset is defined as a set of members all of which are also members of another set.
The definition of subset stated above matches the general use of the word subset in set theory and mathematics. The notes below apply this definition to subsets of SNOMED CT components.
A subset of SNOMED CT concepts is a defined set of concepts taken from a wider set of concepts (e.g. all the concepts in a particular version of a specified SNOMED CT edition).
Similarly, a subset of SNOMED CT descriptions is a set of descriptions taken from a wider set of descriptions (e.g. all the descriptions in a particular version of a specified SNOMED CT edition).
The members of a subset can defined in one of two ways extensionally, by enumeration, or intensionally, using rules to determine inclusion.
The standard distribution format for extensionally defined subsets is a simple reference set, while the standard distribution format for intensionally defined subsets is query reference sets.
The diagram below shows an example of a subset. The English vowels are a subset of the set of alphabet characters included in the English alphabet
Figure 2.1-1: Subset example