Q. Which version of a publication can be openly accessible?

Often, the Submitted Version or Accepted Version of a publication is permitted to be open and uploaded into an institutional repository (e.g. Research Profiles and Repository). 

Typically, authors usually assign their copyright to publishers when they negotiate contracts. However, as a  researcher, it is important to understand the rights/permissions you have as an author

The Submitted version is the preprint of the manuscript paper before peer review. This is the author's version of the article that was sent to a journal for consideration.

The Accepted version is the post-print of the manuscript, paper after peer-review. This is the author's version of an article that has been amended to reflect any revisions in the publication process.

The Published version is the final version as it appears in the publication.

Note: Jisc's open policy finder provides a guide to publisher’s copyright policies.