Topic-Based Authoring
Each topic is a separate XHTML file with an .htm extension. This enables you to take advantage of topic-based authoring. In other words, rather than creating a few very long documents (as you might do when working in a program such as Adobe FrameMaker or Microsoft Word), you create many small documents and then piece those separate files together to produce various outputs.
Individual topic files are stored in the Projects > Files page, within the Content folder. Topics can be stored either at the root level or in custom folders that you create.