# Subsets, Value Sets and Reference Sets

In this section, we define some important terms that are used throughout this guide. In particular, *subset*, *value set* and *reference set*.

*Subset* and *value set* are general terms that are not specific to SNOMED CT. However, it is important to understand what they mean, and how they relate to SNOMED CT *reference sets*.

In summary:

* A **subset** is a set of members all of which are members of another set (from *set theory* in mathematics).
* A **value set** is a uniquely identifiable set of valid concept representations, where any concept representation can be tested to determine whether or not it is a member of the *value set*.
* A **reference set** is a standard format for maintaining and distributing a set of references to SNOMED CT components.

In the following subsections, we describe each of these terms in more detail.

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