> For the complete documentation index, see [llms.txt](https://docs.snomed.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.snomed.org/snomed-ct-practical-guides/snomed-ct-reference-set-guide/2-subsets-value-sets-and-reference-sets.md).
# 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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