You may have noticed that many journal publishers now include a CrossMark icon near the journal title of online articles and their PDF versions. For an example, see:
Paraskevaidi M, Martin-Hirsch PL, Martin FL. Need for early, minimally invasive cancer diagnosis. Proc Natl Acad Sci U S A. 2019 Mar 12;116(11):4752.
doi: 10.1073/pnas.1817551116. Epub 2019 Feb 19. PubMed PMID: 30782793; PubMed Central PMCID: PMC6421421, in HTML and PDF.
By clicking on the CrossMark icon, readers can immediately determine whether or not they are viewing the most up-to-date version of the article, whether updates exist, whether this article has had supplementary data added, has been corrected or retracted, etc. It basically “provides a standard way for readers to locate the authoritative version of a document”, as per one of the participating publishers.
CrossMark is an initiative of CrossRef, a not-for-profit membership organization that creates meta-data in order to make “research outputs easy to find, cite, link, and assess”. Participating member publishers pay a fee to incorporate CrossMark into their journals.