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

CodeDisplayDefinition
fatalFatalThe issue caused the action to fail, and no further checking could be performed.
errorErrorThe issue is sufficiently important to cause the action to fail.
warningWarningThe 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.
informationInformationThe issue has no relation to the degree of success of the action.