Considering the ambiguity of USCDI data classes and data elements and recognizing that the allocation of data elements to data classes based on the actual individual data element's definition, one cannot draw a straight line from USCDI terminology to corresponding FHIR US Core terminology.  Since FHIR US Core is used for certification, while USCDI is used in general industry discussions, a reader of USCDI may anticipate more or different capabilities in their certified HIT when not familiar with FHIR US Core and that FHIR US Core is an interpretation and possibly a subset of one may believe or understand that USCDI is intended to cover.

In the absence of more clarity and less ambiguity in USCDI that can provide a straight-line navigation and interpretation of FHIR US Core, thus what is actually being certified, we suggest that the USCDI page is updated to clearly state that:

• FHIR US Core is used for certification and that any variances in interpretation of USCDI are intended to be resolved and reflected in FHIR US Core.
• that in that process, data elements listed in certain data classes may not appear in a FHIR US Core resource of the same name used for the data class in USCDI.
• or that data classes in USCDI may appear to have a wider scope than the similarly named FHIR resource.
• and that other resources that might by some to be interpreted as a type or category of a USCDI data class are not yet represented in FHIR US Core until USCDI more explicitly expresses that intended scope.
• we therefore encourage to request necessary clarifications to USCDI through the USCDI ONDEC process [USCDI ONDEC | Interoperability Standards Advisory (ISA) (healthit.gov)|https://www.healthit.gov/isa/ONDEC] that will inform future updates to FHIR US Core.

We should also consider whether the same language should be provided as part of the disposition in the corresponding CDA C-CDA CG version while C-CDA documents are intended to remain in sync with FHIR US Core data content.