<article> element can be used to indicate independant content that is self-contained and makes sense in its own right .
This element shows that the content could be re-used or redistributed and its meaning would remain intact - it doesn't rely on the surrounding content.
But in a slightly different vein, when
<article> elements are nested inside each other it shows that the inner elements all have a relationship to the
outer element. An example of this is nesting user comments about an article or blog post inside an
<article> element containing the original article/post.
definition seems to contradict the independant requirement for using the
<article> element, as user comments sometimes refer to previous comments and make no sense
if taken out of context, but it is an example given in the specification.
As is the case with the
<section> element, the
<article> element can contain its own set of headings, including
which don't conflict with the heading hierarchy and the rule to only have one
<h1> heading on a page because they are placed inside a sectioning element.