This code system http://hl7.org/fhir/reference-version-rules defines the following codes:

CodeDisplayDefinition
either Either Specific or independentThe reference may be either version independent or version specific.
independent Version independentThe reference must be version independent.
specific Version SpecificThe reference must be version specific.
<status value="active"/><experimental value="false"/><date value="2021-03-11T17:06:20+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="Whether a reference needs to be version specific or version independent, or whether either can be used."/><caseSensitive value="true"/><valueSet value="http://hl7.org/fhir/ValueSet/reference-version-rules"/><content value="complete"/><concept><code value="either"/><display value="Either Specific or independent"/><definition value="The reference may be either version independent or version specific."/></concept><concept><code value="independent"/><display value="Version independent"/><definition value="The reference must be version independent."/></concept><concept><code value="specific"/><display value="Version Specific"/><definition value="The reference must be version specific."/></concept></CodeSystem>