This code system http://hl7.org/fhir/search-entry-mode defines the following codes:

CodeDisplayDefinitionDeprecated
match MatchThis resource matched the search specification.
include IncludeThis resource is returned because it is referred to from another resource in the search set.
outcome OutcomeAn OperationOutcome that provides additional information about the processing of a search.Deprecated

Use Bundle.issues instead

<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="Why an entry is in the result set - whether it's included as a match or because of an _include requirement, or to convey information or warning information about the search process."/><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/search-entry-mode"/><content value="complete"/><concept><code value="match"/><display value="Match"/><definition value="This resource matched the search specification."/></concept><concept><code value="include"/><display value="Include"/><definition value="This resource is returned because it is referred to from another resource in the search set."/></concept><concept><extension url="http://hl7.org/fhir/StructureDefinition/structuredefinition-standards-status"><valueCode value="deprecated"><extension url="http://hl7.org/fhir/StructureDefinition/structuredefinition-standards-status-reason"><valueMarkdown value="Use Bundle.issues instead"/></extension></valueCode></extension><code value="outcome"/><display value="Outcome"/><definition value="An OperationOutcome that provides additional information about the processing of a search."/></concept></CodeSystem>