Section vs Article HTML5

In the W3 wiki page about structuring HTML5, it says:

<section>: Used to either group different articles into different purposes or
subjects, or to define the different sections of a single article.

And then displays an image that I cleaned up:

It also describes how to use the <article> tag (from same W3 link above):

<article> is related to <section>, but is distinctly different.
Whereas <section> is for grouping distinct sections of content or
functionality, <article> is for containing related individual
standalone pieces of content, such as individual blog posts, videos,
images or news items. Think of it this way – if you have a number of
items of content, each of which would be suitable for reading on their
own, and would make sense to syndicate as separate items in an RSS
feed, then <article> is suitable for marking them up.

In our example, <section id="main"> contains blog entries. Each blog
entry would be suitable for syndicating as an item in an RSS feed, and
would make sense when read on its own, out of context, therefore
<article> is perfect for them:

<section id="main">
    <article>
      <!-- first blog post -->
    </article>

    <article>
      <!-- second blog post  -->
    </article>

    <article>
      <!-- third blog post -->
    </article>
</section>

Simple huh? Be aware though that you can also nest sections inside
articles, where it makes sense to do so. For example, if each one of
these blog posts has a consistent structure of distinct sections, then
you could put sections inside your articles as well. It could look
something like this:

<article>
  <section id="introduction">
  </section>

  <section id="content">
  </section>

  <section id="summary">
  </section>
</article>