From the NLM Technical Bulletin:
Revisions, scientific updates, and updates of reviews are examples of content that could be versioned. Versions are not intended for correcting specific errors in an article, for which published errata notices should continue to be used.
Version information is noted on the Summary, Summary (text), Abstract, and Abstract (text), displays. On the Summary display, the version number is located before the journal title abbreviation. The article’s original publication date is retained, and the version publication date is enclosed in square brackets.