A recent report from HEFCE states that in order to be eligible for the post-2014 REF, authors’ final peer-reviewed manuscripts must have been deposited in an institutional or subject repository on acceptance for publication, and must be discoverable (and free to read) by anyone with an internet connection.
The requirement applies only to journal articles and conference proceedings with an International Standard Serial Number. It will not apply to monographs, book chapters, other long-form publications, working papers, creative or practice-based research outputs, or data. Although the policy applies to research outputs accepted for publication after 1 April 2016, it is strongly advised to implement the policy now.
For more details, please see the full report here.