Seeking clarification on HTML sectioning elements and terminology in the specification

Hi.

I'm having difficulty understanding HTML specs, specifically regarding
sections. This probably has to do with an interpretation problem of mine,
but since I couldn't solve it, I decided to contact the community.

Section 4.3 in the specs is named "Section", and discusses body, article,
section, nav, aside, h1-h6, hgroup, header, footer, and address elements.
This makes me think that all those elements mentioned are used to mark a
section.

However, that seems a bit contradictory because only a subset of those
elements is categorized as "sectioning content", which are the article,
section, aside, and nav.

The answer to the following two questions would clarify matters for me:

What is the meaning of the term "section" in the specification?
Which elements are intended to mark up a section?