This code system http://hl7.org/fhir/encounter-reason-use defines the following codes:

Code Display
CC Chief Complaint
HC Health Concern
AD Admitting Diagnosis
RV Reason for Visit
HM Health Maintenance (including screening)
<status value="active"/> <experimental value="false"/> <date value="2021-01-05T10:01:24+11:00"/> <publisher value="HL7 (FHIR Project)"/> <contact> <telecom> <system value="url"/> <value value="http://hl7.org/fhir"/> </telecom> <telecom> <system value="email"/> <value value="fhir@lists.hl7.org"/> </telecom> </contact> <description value="Encounter Reason Use"/> <jurisdiction> <coding> <system value="http://unstats.un.org/unsd/methods/m49/m49.htm"/> <code value="001"/> <display value="World"/> </coding> </jurisdiction> <caseSensitive value="true"/> <valueSet value="http://hl7.org/fhir/ValueSet/encounter-reason-use"/> <content value="complete"/> <concept> <code value="CC"/> <display value="Chief Complaint"/> </concept> <concept> <code value="HC"/> <display value="Health Concern"/> </concept> <concept> <code value="AD"/> <display value="Admitting Diagnosis"/> </concept> <concept> <code value="RV"/> <display value="Reason for Visit"/> </concept> <concept> <code value="HM"/> <display value="Health Maintenance (including screening)"/> </concept> </CodeSystem>