Self-archiving, also known as green open access, refers to the practice of depositing articles in an institutional repository or a subject repository such as arXiv. Green open access journal publishers endorse immediate open access self-archiving by their authors. (Wikipedia).
A second (better) way authors can make their work open access is by publishing it in such a way that makes their research output immediately available from the publisher. This is known as 'gold' open access. JMIR Publications is a gold open access publisher. The publisher does all the work in archiving (e.g. submission to PubMed Central). Usually the author does not need to "self-archive" because the paper is already openly accessible upon publication, however, as JMIR authors retain the copyright, he is legally allowed to do so (e.g. deposit the copy in an institutional repository or wherever else he likes), without having to obtain a permission from JMIR Publications. Thus, one could say that all JMIR journals are also "green" journals (see Sherpa/RoMEO database), because "gold trumps green" (all gold open access journals are automatically also "green" journals because they obviously allow self-archiving).
However, a caveat on self-archiving: Note that in the case of an article correction, authors are responsible for updating/correcting copies they self-archived (which is another advantage of gold vs green open access publishing). JMIR Publications will correct known copies of the article in databases such as PubMed Central, but it is the authors' responsibility to curate/update an article in institutional repositories should they decide to deposit it. This is why we recommend to self-archive only if your institution mandates it.