Since 1st January 2018, Crossref has had a new reference distribution policy, described at https://www.crossref.org/reference-distribution/.
There are three possible options for setting the reference distribution preference from which a publisher can choose, these being ‘Closed’, ‘Limited’ and ‘Open”.
If the ‘Closed’ option is chosen, the references will only be used for the Crossref Cited-by service, and are not distributed via any of the other Crossref interfaces.
If the ‘Limited’ option is chosen, the references will additionally be made available to Crossref metadata subscribers who have signed an agreement for the new Crossref Metadata APIs ‘Plus’ service which also came into effect on 1st January 2018. This is the new Crossref default option.
If the ‘Open’ option is chosen, the references will be openly available to anyone using the Crossref APIs.
Publishers will no longer be able to select the reference distribution preference for individual publications on a case-by-case basis, but rather the preference will be set for all publications appearing under a particular DOI prefix, with the new default option being ‘Limited’.
To date, more than 60 scholarly publishers, including most of the major ones, have responded to the Initiative for Open Citations and have instructed Crossref to set their references to open, as describe in a previous post.