This page will list all known external ID prefixes that might appear in the
externalIds values for a document. These are intended to capture the ones we know about, but should not be taken as an exhaustive list. It's better for data integration to keep an ID we don't know that we need with a custom/temporary prefix, even if it winds up going unused in the long run.
pmid: A PMID value, which is simply a large integer.
pmcid: A PMCID value, which is simply a large integer prefixed by "PMC."
Every paper indexed by PubMed should in principle have a PMID value, while only papers that are deposited in PubMed Central (the full-text open access repository) have PMCID values.
pmmid: A PubMed Manuscript ID value, which indicates the way in which that manuscript was deposited into PubMed Central.
For more on these first three types of values and a tool to manually convert between them (when possible), see this page at PubMed Central.
pii: A publisher-specific identifier that may or may not follow the standard PII format.
scopus: A Scopus ID value, which is simply a large integer.
sici: a Serial Item and Contribution Identifier that may or not follow the standard SICI format.
semanticScholar: a Semantic Scholar identifier, which is a 40-character hex string.
orcid: An ORCID identifier, of the form "1234-1234-1234-1234".
researcherid: A ResearcherID identifier, of the form "X-1234-5678".