Preamble: If you wish to sample some of the most absurd, incoherent, pseudo-legal gibberish on the subject of "rights" retention, "systematicity" and free will, please have a look at what follows under "Elsevier Article Posting Policies" below. (And bear in mind that an institution only provides a tiny fraction of any journal's content.) 

Any author foolish enough to be intimidated by this kind of garbled double-talk deserves everything that's coming to him. 

My Advice to Authors: Ignore this embarrassing, self-contradictory nonsense completely and exercise your retained "right" to post your final refereed draft ("AAM") in your institutional repository immediately upon acceptance, whether or not it is mandatory, secure in the knowledge that from a logical contradiction anything and everything (and its opposite) follows! (And be prepared to declare, with hand on heart, that as an adult, every right you exercise with your striate musculature is exercised "voluntarily.") 

[By the way, as long as Elsevier states that its authors retain the right to post "voluntarily", Elsevier, too, remains on the Side of the Angels insofar as immediate, unembargoed Green OA self-archiving is concerned. It's just that the Angels are a bit glossalalic...]

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Elsevier Article Posting Policies

Accepted author manuscripts (AAMs)

Definition: An accepted author manuscript (AAM) is the author's version of the manuscript of an article that has been accepted for publication and which may include any author-incorporated changes suggested through the processes of submission processing, peer review, and editor-author communications. AAMs do not include other publisher value-added contributions such as copy-editing, formatting, technical enhancements and (if relevant) pagination.

Policy: Authors retain the right to use the accepted author manuscript for personal use, internal institutional use and for permitted scholarly posting provided that these are not for purposes of commercial use or systematic distribution...

Permitted scholarly posting: Voluntary [emphasis added] posting by an author on open websites operated by the author or the author's institution for scholarly purposes, as determined by the author, or (in connection with preprints) on preprint servers...

...Elsevier believes that individual authors should be able to distribute their AAMs for their personal voluntary [emphasis added] needs and interests, e.g. posting to their websites or their institution's repository, e-mailing to colleagues. However, our policies differ regarding the systematic aggregation or distribution of AAMs to ensure the sustainability of the journals to which AAMs are submitted [emphasis added]. Therefore, deposit in, or posting to, subject-oriented or centralized repositories (such as PubMed Central), or institutional repositories with systematic posting mandates [emphasis added] is permitted only under specific agreements between Elsevier and the repository, agency or institution, and only consistent with the publisher's policies concerning such repositories. Voluntary [emphasis added] posting of AAMs in the arXiv subject repository is permitted.

Systematic distribution means: policies or other mechanisms designed to aggregate and openly disseminate, or to substitute for journal-provided services, including:

--- The systematic distribution to others via e-mail lists or list servers (to parties other than known colleagues), whether for a fee or for free...

--- Institutional, funding body or government manuscript posting policies or mandates that aim to aggregate and openly distribute the work by its researchers or funded researchers...