In addition to creating new topics into which you enter new content you can also create TOC entries that display content from other sources when the user clicks on the TOC entry.
You can create TOC entries that link to web pages or to topics in other help files that are available when the user views the help.
You can also insert entire Help+Manual projects in your TOC. When you publish your project the external projects are merged with the main project.
You can also use Snippets to insert topics and XML files from other sources into existing topics. Snippets can be inserted as editable copies or linked to the source files so that they update automatically when the sources are edited.