This code system http://hl7.org/fhir/concept-map-relationship defines the following codes:

Lvl Code Display Definition
1 related-to Related To The concepts are related to each other, but the exact relationship is not known.
2   equivalent Equivalent The definitions of the concepts mean the same thing.
2   source-is-narrower-than-target Source Is Narrower Than Target The source concept is narrower in meaning than the target concept.
2   source-is-broader-than-target Source Is Broader Than Target The source concept is broader in meaning than the target concept.
1 not-related-to Not Related To This is an explicit assertion that the target concept is not related to the source concept.
<status value="draft"/> <experimental value="false"/> <date value="2023-03-26T15:21:02+11:00"/> <description value="The relationship between concepts."/> <jurisdiction> <coding> <system value="http://unstats.un.org/unsd/methods/m49/m49.htm"/> <code value="001"/> <display value="World"/> </coding> </jurisdiction> <caseSensitive value="true"/> <valueSet value="http://hl7.org/fhir/ValueSet/concept-map-relationship"/> <content value="complete"/> <concept> <code value="related-to"/> <display value="Related To"/> <definition value="The concepts are related to each other, but the exact relationship is not known."/> <concept> <code value="equivalent"/> <display value="Equivalent"/> <definition value="The definitions of the concepts mean the same thing."/> </concept> <concept> <code value="source-is-narrower-than-target"/> <display value="Source Is Narrower Than Target"/> <definition value="The source concept is narrower in meaning than the target concept."/> </concept> <concept> <code value="source-is-broader-than-target"/> <display value="Source Is Broader Than Target"/> <definition value="The source concept is broader in meaning than the target concept."/> </concept> </concept> <concept> <code value="not-related-to"/> <display value="Not Related To"/> <definition value="This is an explicit assertion that the target concept is not related to the source concept."/> </concept> </CodeSystem>