Initially, Crossref metadata did not include the citations from an article, but from 2004 8 these were added as a discrete component in the form of structured citations. Initial uptake by publishers was slow, but nowadays it is almost universal. These structured citations of books and journal articles included conventional information such as the author and journal name and the volume and page numbers, but in time these evolved to also include the article DOI, which allows facile and programmatic access to the metadata record for each citation. At this stage a record is introduced for one specific article and its access point in the form suitable for AI/ML applications: .crossref.unixsd+xml An example of a structured citation from this record (as of mid 2024) is shown below: <citation key="D3DD00246B/cit25/1"> <journal_title>J. Chem. Phys.</journal_title> <author>Scalmani</author> <cYear>2010</cYear> <first_page>114110</first_page> <doi>10.1063/1.3359469</doi> </citation> If you explore the metadata further, you will soon encounter a slightly diYerent form, which is designated an unstructured citation, arising by virtue of inclusion of a component containing free-text comments. This is how all those citation footnotes, comments and other annotations so beloved by some authors are currently included. In this example, the article DOI itself is also noted, thus rendering the unstructured component somewhat redundant, but this is not always the case! Here one might infer from the volume title that this is now about data. This is a suitable entry point for the discussion here to rejoin the theme introduced above regarding data and ESI. However, instead of referring to data inside an ancillary PDF file associated with the article, a data DOI is now cited instead. As implied above for article DOIs, this form also has an associated metadata record, being stored, curated and disseminated by DataCite, an organisation set up some ten years after Crossref but acting in parallel to allow the citation of data. Unlike data contained in relatively unstructured -or parochially structured ESI documents, this form of data has associated formal descriptors in the metadata record describing the properties of the data. DataCite also allow access to this record, albeit using a slightly diYerent form to that used by Crossref: c/13108