The format tools at the top of your page provide you with various ways to edit and format your text (i.e. change the font, add special characters, manage indentations, etc). Along the side, you have the toolbox to interactive objects to your chapter. When editing a chapter, your interactive objects will be replaced with placeholders showing information about the element. To have an actual view of the interactive object in action, click on the Preview mode button at the bottom right. 


At the footer of the page, it will mention the type of layout this chapter is using: reflowable or fixed layout. You will also find a number of options, depending on the nature of the chapter.

Invisible This option will hide the chapter entirely from the EPUB's spine. The only way for the reader to reach this chapter, is by clicking on a hyperlink that links to it.
Show content This option will only display if the current chapter contains subchapters. Once the user selects this chapter in your book, it can either show its own content, or be treated only as a folder to display its subchapters. By dedault, the chapter content will be shown.
Include in TOC This option will remove this chapter from the table of contents, but keep it in the ebook's regular navigation. Users may reach this chapter by navigating with next and previous buttons. This option is wise for chapters such as appendix, bibliography, etc.
Width (Fixed layouts only) The viewport/page width for the chapter.
Height (Fixed layouts only) The viewport/page height for the chapter.
Grid (Fixed layouts only) To help with design and alignments, display a grid. Once checked, you can select the size of the grid. This grid will not display to the user, but only to you as the author.
Snap (Fixed layouts only) Restrict the positioning of elements to the grid lines. This option is displayed only if Grid is checked.

Note: To prevent loss of your work, it is recommended to periodically save your file, preferably with the date appended to the filename.

For more details on adding interactive objects, refer to the Interactive objects knowledge-base category. For information on adding direct HTML, JS, or CSS to your chapter, refer to the Adding custom interactivity knowledge-base article.