This code system http://hl7.org/fhir/sort-direction defines the following codes:

CodeDisplayDefinition
ascending AscendingSort by the value ascending, so that lower values appear first.
descending DescendingSort by the value descending, so that lower values appear last.
<status value="draft"/><experimental value="false"/><date value="2023-03-26T15:21:02+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="The possible sort directions, ascending or descending."/><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/sort-direction"/><content value="complete"/><concept><code value="ascending"/><display value="Ascending"/><definition value="Sort by the value ascending, so that lower values appear first."/></concept><concept><code value="descending"/><display value="Descending"/><definition value="Sort by the value descending, so that lower values appear last."/></concept></CodeSystem>