The article element specifies independent, self-contained content. It should be possible to distribute it independently and it still make sense. A blog post, news article, etc. Its content can be divided by sections, <section> and have its own heading, it can also contain its own header footer.
ARIA role attribute value: depends on content
The article element represents a complete, or self-contained, composition in a document, page, application, or site and that is, in principle, independently distributable or reusable, e.g. in syndication. This could be a forum post, a magazine or newspaper article, a blog entry, a user-submitted comment, an interactive widget or gadget, or any other independent item of content.HTML 5.1 Nightly
When article elements are nested, the inner article elements represent articles that are in principle related to the contents of the outer article. For instance, a blog entry on a site that accepts user-submitted comments could represent the comments as article elements nested within the article element for the blog entry.
Author information associated with an article element (q.v. the address element) does not apply to nested article elements.
When used specifically with content to be redistributed in syndication, the article element is similar in purpose to the entry element in Atom. [ATOM]
The schema.org microdata vocabulary can be used to provide the publication date for an article element, using one of the CreativeWork subtypes.
When the main content of the page (i.e. excluding footers, headers, navigation blocks, and sidebars) is all one single self-contained composition, the content should be marked up with a main element and the content may also be marked with an article, butit is technically redundant in this case (since it's self-evident that the page is a single composition, as it is a single document).