PMID vs PMCID: What’s the Difference?
(Paraphrased from NIH)
PubMed is an index of the biomedical literature. A PMID, also known as the PubMed reference number, is a number assigned by the NIH National Library of Medicine to papers indexed in PubMed. PubMed Central is an archive of full-text journal articles. The National Library of Medicine assigns a PMCID, also known as a PMC Identifier, to each full-text paper in PubMed Central.
To comply with the public access policy, anyone submitting an application, proposal or report to the NIH must include a PMCID when citing applicable papers that they author or that arise from their NIH-funded research. If it helps to remember, think of the C in PMCID to indicate compliance with the public access policy.
While all abstracts appearing in PubMed are assigned a PMID automatically, NIH awardees may need to take action to obtain a PMCID.
You need to ensure papers are compliant with the public access policy as soon as the paper is accepted for publication. Try this interactive wizard to determine if your paper falls under the policy, how to bring it into compliance, and how to report it to NIH.