This code system http://hl7.org/fhir/namingsystem-identifier-type defines the following codes:

CodeDisplayDefinition
oidOIDAn ISO object identifier; e.g. 1.2.3.4.5.
uuidUUIDA universally unique identifier of the form a5afddf4-e880-459b-876e-e4591b0acc11.
uriURIA uniform resource identifier (ideally a URL - uniform resource locator); e.g. http://unitsofmeasure.org.
otherOtherSome other type of unique identifier; e.g. HL7-assigned reserved string such as LN for LOINC.