HTML5 spec说(Section)：

The section element represents a generic section of a document or
application. A section,in this context,is a thematic grouping of
content,typically with a heading. […]

Examples of sections would be chapters,the varIoUs tabbed pages in
a tabbed dialog Box,or the numbered sections of a thesis. A Web
site’s home page could be split into sections for an introduction,
news items,and contact information.

注意：鼓励作者使用article元素而不是
节元素，当它有意义的联合的内容
元素。

并为Article

The article element represents a 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.

我认为你在OP中称为“部分”符合文章的定义，因为我可以看到他们是可独立分配或可重复使用。

更新：latest editors draft for HTML 5.1中文章的一些细微文本更改(更改为斜体)：

The article element represents a complete,or self-contained,
composition in a document,in
principle,independently distributable or reusable,e.g. in
syndication. This could be a forum post,a magazine or newspaper
article,a user-submitted comment,an interactive widget
or gadget,or any other independent item of content.

此外，关于2013年January和February期间关于文章的公共HTML邮件列表的讨论。