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

CodeDisplayDefinition
routineRoutineThe request has normal priority
urgentUrgentThe request should be actioned promptly - higher priority than routine
asapASAPThe request should be actioned as soon as possible - higher priority than urgent
statSTATThe request should be actioned immediately - highest possible priority. E.g. an emergency