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

CodeDisplayDefinition
normal NormalThe search parameter is derived directly from the selected nodes based on the type definitions.
phonetic PhoneticThe search parameter is derived by a phonetic transform from the selected nodes.
other OtherThe interpretation of the xpath statement is unknown (and can't be automated).
<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="How a search parameter relates to the set of elements returned by evaluating its expression query."/><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-processingmode"/><content value="complete"/><concept><code value="normal"/><display value="Normal"/><definition value="The search parameter is derived directly from the selected nodes based on the type definitions."/></concept><concept><code value="phonetic"/><display value="Phonetic"/><definition value="The search parameter is derived by a phonetic transform from the selected nodes."/></concept><concept><code value="other"/><display value="Other"/><definition value="The interpretation of the xpath statement is unknown (and can't be automated)."/></concept></CodeSystem>