There is a big change in what goes into an output in Adobe RoboHelp 2019. In earlier versions, an output contained everything that was in Project Manager that was not excluded by the build expression. In Adobe RoboHelp 2019 it is controlled by the table of contents.
Every
topic that you want in the output must be in the TOC or have a link from
a topic that is in the TOC.
You may not want users to see a TOC with every topic that can be accessed, some you may just want to be links from other topics. Not every topic has to be included in the TOC. Let's say you have a topic that has links to say twenty others that give supplementary information. Only that one topic needs to be in the TOC as the links will automatically pull in the twenty supplementary topics. That does not prevent you from including any or all of the twenty, it's just that you don't have to.
Because the TOC controls what goes into an output, you also use it to add baggage files to the output. Baggage files are files you want in the build when you generate but that are not referenced anywhere.
When you click Insert Pages while editing a TOC, as shown below in Adding Items to the TOC, there will be an option Local Files. Select the baggage file you want to add and it will be uploaded when you generate. If you don't want the baggage file to appear in the TOC, just tick Hide in Output in the Properties panel.
To see the Table of Contents in an upgraded project or to create a new
one, click the TOC icon in the left toolbar.

If you right click in the Table of Contents Panel shown, the options
below will be shown and you can edit in that area.

If you prefer a larger area to work in, click the menu icon and select
edit. The TOC will open in the authoring area to the right and icons shown
above will appear in the top toolbar. The TOC Properties will be displayed
in the Right Panel.

Click New Page to display this dialog.

Note the options. You can add Project Files, Local Files outside your project
(Baggage Files), links to Web Pages and create draft emails for your users.
Creating emails is new. The
title will appear in the TOC. Clicking that page will open the email in
the users default email program.
