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

CodeDisplay
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>