Card.source.topic was introduced in CDS Hooks STU2. STU2 is being published (any day now, here: https://cds-hooks.hl7.org/).

CRD SHALLs CRD services support of this this data element:

Card.source.topic SHALL be populated, and has an extensible binding to the ValueSet CRD Card Types. The rationale is to allow EHRs to potentially filter or track the usage of different types of cards.

http://hl7.org/fhir/us/davinci-crd/2022May/hooks.html#general-guidance

The card.source.topic is defined as a coding data type, see: https://cds-hooks.org/specification/current/#coding. This was identified by an implementer as a desirable feature because it simplified data type handling and introduced some consistency, for full history, see: https://github.com/cds-hooks/docs/pull/531

Currently, compliant CDS Hooks clients and services are stuck deciding if they should follow CDS Hooks or CRD for this data element.

CRD should change the data type used for card.source.topic to coding as defined in CDS Hooks STU2.