Contrary to what one might expect, not all the items in open access repositories are publicly available. Estimates of the percentage of the content in repositories that is not in fact open access tend to range from around 40% to 60%. This will include bibliographic records containing only metadata, plus full-text documents that have been placed on “dark deposit” — i.e. documents that are present in the repository but not freely available, either because they are subject to a publisher’s embargo or because the author(s) asked for the full-text to be deposited on a closed access basis. To enable researchers to nevertheless obtain copies of items that have been placed on dark deposit OA advocates developed the request eprint button. But how does the button work, and how effective is it?
Eloy Rodrigues, Director of Documentation Services at the University of Minho, discusses the issues and outlines the situation at UMinho here: http://poynder.blogspot.co.uk/2015/03/open-access-and-request-eprint-button-q.html