the Encounter resource does not specify Principal Diagnosis which is a US Realm requirement to address the condition that, after study, represents the condition that is responsible for the hospital admission. After several discussions with the Patient Administration, Patient Care and Financial Work Groups, the advice was to define Principal Diagnosis by constraining Encounter.diagnosis.role to "Billing Diagnosis" and requiring Encounter.diagnosis.rank =1. That is the current approach used by US efforts to create electronic clinical quality measures (eCQMs) for implementation in government programs, and in CDS artifacts for testing. The CQI Work Group is mapping the CMS Quality Data Model Encounter attribute Principal Diagnosis to QI Core in the manner indicated. The issue is not universal domain since the concept "Principal Diagnosis" is unique to the US. However, the US Core should include reference to these constraints such that system implementers are able to consistently apply the FHIR resources and retain consistent meaning.