This code system http://terminology.hl7.org/CodeSystem/citation-variant-type defines the following codes:

CodeDisplayDefinition
version versionDenotes a specific version of an article or artifact.
subpart subpartDenotes a specific portion of an article, such as page, section, paragraph, line, table or figure.
supplement Supplement or AppendixUsed to denote a supplementary file, appendix, or additional part that is not a subpart of the primary article.
supplement-subpart Supplement or Appendix SubpartUsed to denote a subpart within a supplementary file or appendix.
base-for-article-set Base citation for an article setUsed to define the base citation or an article set with individual articles as variant citations.
article-set Part of an article setUsed to distinguish an individual article within an article set where the article set is a base citation.
<status value="draft"/><experimental value="false"/><date value="2020-08-20T17:41:31+10:00"/><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="To describe the reason for the variant citation, such as version number or subpart specification."/><caseSensitive value="true"/><valueSet value="http://hl7.org/fhir/ValueSet/citation-variant-type"/><content value="complete"/><concept><code value="version"/><display value="version"/><definition value="Denotes a specific version of an article or artifact."/></concept><concept><code value="subpart"/><display value="subpart"/><definition value="Denotes a specific portion of an article, such as page, section, paragraph, line, table or figure."/></concept><concept><code value="supplement"/><display value="Supplement or Appendix"/><definition value="Used to denote a supplementary file, appendix, or additional part that is not a subpart of the primary article."/></concept><concept><code value="supplement-subpart"/><display value="Supplement or Appendix Subpart"/><definition value="Used to denote a subpart within a supplementary file or appendix."/></concept><concept><code value="base-for-article-set"/><display value="Base citation for an article set"/><definition value="Used to define the base citation or an article set with individual articles as variant citations."/></concept><concept><code value="article-set"/><display value="Part of an article set"/><definition value="Used to distinguish an individual article within an article set where the article set is a base citation."/></concept></CodeSystem>