The Equivalent Code Sequence (0008,0121) attribute may optionally be used to convey different codes for the same concept.
Some scenarios in which is its helpful for the creating system to send equivalent codes include:
when different representations of the same concept are present in a standard coding scheme, such as the SNOMED-CT and SNOMED-RT and CTV3 style identifiers,
when the same concept is present in different standard coding schemes, but considered by the creating system to be synonymous, such as anatomical concepts from SNOMED and FMA, and
when the same concept is present in a local as well as a standard coding scheme, but considered by the creating system to be synonymous, such as a local private procedure code and the same concept in LOINC or SNOMED or RADLEX.