In fact, according to the specifications, defining_code should only be used if the DV_CODED_TEXT.value is exactly the rubric of the terminology code. Otherwise, DV_TEXT and mappings must be used.
Actually I think that is incorrect, especially now that we can also carry preferred term. We frequently have situations where we have modelled e.g a list of SNOMED terms but the clinicians want a slightly different UI. As long as that is done under ‘informatics control’ I think ti is more correct to carry the SNOMED code as the defining_code but alongside the text value. That is also identical (IMO!) to the CodeableConcept approach in FHIR