The Formatting_Guidelines state:
Section headers are usually obvious as Titles centered between paragraphs of a page. Check the Table of Contents if in doubt. This is often linked to on the Project page, otherwise find the contents page by clicking on Images, Pages Proofread & Differences on the Project page. The page you are looking for will probably been among the first ten .png images in the first column. Project managers(PMs) will sometimes specify exceptions in the Project Comments.
Ads sometime require creative use of section headers to describe centered text.
The Formatting Guidelines state:
Some books have sections within chapters. Format these headings as they appear in the image. Leave 2 blanks lines before the heading and one after, unless the Project Manager has requested otherwise. If you are not sure if a heading indicates a chapter or a section, post a question in the Project Discussion, noting the page number.
Mark any italics or mixed case small caps that appear in the image. While section headings may appear to be bold or spaced out, these are usually the result of font or font size changes and should not be marked. The extra blank lines separate the heading, so do not mark the font change as well.
For more: see here.