The PMID is the unique identification number that PubMed assigns for each publication that is entered into this database of abstracts.
The PMID has nothing to do with the NIH Public Access Policy.
The PMCID, on the other hand, is part of the policy. PubMed Central (PMC)
Each full text publication is:
For further information about the actual policy see NIH Public Access Policy details.
An embargo period means there is a delay of up to 12 months following the publication of an article before the full text is made available in PubMed Central. This period is set by the publisher of the journal and is included in the copyright transfer agreement or an addendum. If you add the publication with an embargo into The NIH Manuscript Submission System, the publication will be tracked and then moved to PubMed Central at the appropriate time.