Value	Meaning
accession	(Bioinformatics) a unique identifier given to a DNA or protein sequence record for tracking the sequence record and the associated sequence over time in a data repository.
archive	Generic identifier type for an identifier assigned by an archive or other repository (generally used with elements <object-id>, <issue-id>, and <volume-id>).
ark	Archival Resource Key — a Uniform Resource Locator (URL) containing the word ark that is a multi-purpose identifier for information objects of any type.
art-access-id	Generic article accession identifier for interchange and retrieval between archives.
arxiv	arXiv archive of electronic preprints.
coden	Obsolete PDB/CCDC identifier (may be present on older articles).
custom	Not one of the meanings named in this list. The type can be found as the value of the attribute @custom-type.
doaj	Directory of Open Access Journals.
doi	Digital Object Identifier.
handle	Handle identifier, part of the Handle System for assigning, managing, and resolving persistent identifiers for digital objects and other resources on the Internet.
index	Generic identifier type for an identifier assigned by an abstracting or indexing service (generally used with elements <object-id>, <issue-id>, and <volume-id>).
isbn	International Standard Book Number.
manuscript	Identifier assigned to a manuscript.
medline	NLM Medline identifier.
mr	Mathematical Reviews (MR) identifier
other	None of the named identifiers in this list.
pii	The original Publisher Item Identifier (PII) was a system to provide unique and concise identification for individual published documents with a highly specified string. The PII specification is no longer in common use (2010).
pmcid	PubMed Central identifier.
pmid	PubMed ID; see https://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed.
publisher-id	Publisher’s identifier, such as an article-id, artnum, identifier, article-number, pub-id, etc.
sici	Serial Item and Contribution Identifier. (An older ANSI/NISO Z39.56 code to uniquely identify volumes, journal articles, or other parts of a periodical. A journal article may have more than one SICI, for example, one for a print version and another for an electronic version.)
std-designation	The official number of a standard, from a standards body such as ISO, NISO, IEEE, ASME, and others, for example, Z39.96-2015.
zbl	Zentralblatt MATH (zbMATH) identifier.

Value	Meaning
Text, numbers, or special characters	The type of identifier of the cited objects, which may be a standard, book, journal article, etc. This attribute is typically used to name the type of identifier (such as doi) or the name of the organization or type of organization that created the identifier (ISO or archive).

Value	Meaning
Text, numbers, or special characters	The type of identifier assigned to an internal structure such as a figure or boxed text (typically, a doi, but the attribute may name the organization that created the identifier, CEN, or the type of organization that created the identifier,reseller).