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

CodeDisplayDefinition
fatal FatalThe issue caused the action to fail and no further checking could be performed.
error ErrorThe issue is sufficiently important to cause the action to fail.
warning WarningThe issue is not important enough to cause the action to fail but may cause it to be performed suboptimally or in a way that is not as desired.
information InformationThe issue has no relation to the degree of success of the action.
<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 the issue affects the success of the action."/><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/issue-severity"/><content value="complete"/><concept><code value="fatal"/><display value="Fatal"/><definition value="The issue caused the action to fail and no further checking could be performed."/></concept><concept><code value="error"/><display value="Error"/><definition value="The issue is sufficiently important to cause the action to fail."/></concept><concept><code value="warning"/><display value="Warning"/><definition value="The issue is not important enough to cause the action to fail but may cause it to be performed suboptimally or in a way that is not as desired."/></concept><concept><code value="information"/><display value="Information"/><definition value="The issue has no relation to the degree of success of the action."/></concept></CodeSystem>