Section Editors
Unlike the Entire Page Editor, Section Editors will only display the portion of the web page where the main content exists1. The HTML <head> and any other <div> sections related to navigation or the banner are hidden from you. This allows you to concentrate solely on the specific page content.
There are also some additional options when using the section editors, though they really only apply to the Topic pages. They are the General and Open Graph settings for that page.
1 You probably won't need to worry about this, but in case you're interested:
For normal Topic pages, this would be the area between the <div id="main"> tags.
For the Banner, it is everything between the <body> tags.
