Are questions about a reference system for academic documents off-topic?
Example, in reference to this question:
PubMed IDs (a.k.a. PMIDs) seem to have been assigned sequentially:
However, some some PubMed IDs missing. I.e. https://www.ncbi.nlm.nih.gov/pubmed/2260 returns
Error occurred: The following PMID is not available: 2260. Why are some PubMed IDs missing?