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A map is an association between codes from one code system and codes from another code system, that have the same (or similar) meaning. Mapping is the process of defining a set of maps. Maps are developed in accordance with a documented rationale, for a given purpose. As a result, there may be different maps between the same pair of concepts or terms to meet different use cases.

The purpose of mapping between SNOMED CT and another code system is to provide a link between the code systems, to obtain a number of benefits. These may include:

Data reuse - for example, SNOMED CT based clinical data can be reused to report statistical and management information using an alternative classification system
Retaining the value of existing data when migrating to newer database formats and code systems
Avoiding the need to enter data multiple times and preventing the associated cost and potential errors
Promoting interoperability between terminologies, classifications and code systems

Requirements for a set of maps between SNOMED CT and another code system

Requirement	Description	Example Use	Reference Set
An equivalence map	A set of one-to-one bidirectional maps between SNOMED CT components and codes from another code system	Mapping legacy codes to equivalent SNOMED CT concepts	Simple map reference set
A non-equivalence map	A set of maps from SNOMED CT concepts to codes in another code system, where the map may include: one-to-many or many-to-one maps map groups map rules map advice	Representing a map from SNOMED CT to a statistical classification	Complex and extended map reference sets
	A set of maps from another code system to SNOMED CT, where the map may include: one-to-many or many-to-one maps map groups map rules map advice	Representing a map from a statistical classification to SNOMED CT	Complex and extended map reference sets
	A set of links between codes in another code system and SNOMED CT expressions	Representing the link between LOINC codes and SNOMED CT Expressions	Customized reference set