See QI-Core tracker items #22795 and #22796:

22795 requests an extension in QI-Core to allow expression of a procedure specifically tied to an Encounter (as available for Encounter,diagnosis with metadata to allow for Rank - the intent is to allow reference to a principal procedure associated with an encounter. This concept should be part of the FHIR base resource.

22796 requests another extension in QI-Core to allow reference to a diagnosis that is present on admission. While the Claim resource allows for onAdmission, the Encounter resource does not reference it and the OnAdmission indicator may be part of how care is delivered.