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

CodeDisplayDefinition
ok OKThe message was accepted and processed without error.
transient-error Transient ErrorSome internal unexpected error occurred - wait and try again. Note - this is usually used for things like database unavailable, which may be expected to resolve, though human intervention may be required.
fatal-error Fatal ErrorThe message was rejected because of a problem with the content. There is no point in re-sending without change. The response narrative SHALL describe the issue.
<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="The kind of response to a message."/><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/response-code"/><content value="complete"/><concept><code value="ok"/><display value="OK"/><definition value="The message was accepted and processed without error."/></concept><concept><code value="transient-error"/><display value="Transient Error"/><definition value="Some internal unexpected error occurred - wait and try again. Note - this is usually used for things like database unavailable, which may be expected to resolve, though human intervention may be required."/></concept><concept><code value="fatal-error"/><display value="Fatal Error"/><definition value="The message was rejected because of a problem with the content. There is no point in re-sending without change. The response narrative SHALL describe the issue."/></concept></CodeSystem>