An xref is a reference to an external resource, most often a database entry. Ideally, it should be accessible on the web. The reference consists of two parts: The external database and the accession code for the entry in it.
In those cases where the external database provides a URL template, the Publications system can be configured to allow the button in the interface to open a new browser tab for the database entry.
There is currently also an optional free-text description field associated with an xref. This may be used for a brief characterization of the data available in the database entry.
The procedure to add xrefs to a publication is described in Add dataset xrefs.