In versions of the help file builder prior to version 188.8.131.52, the additional content properties were used to add files to the project to define additional help content that would appear in the table of contents and to add other items such as image files and additional stylesheets. As an alternative to basic HTML files, you could also create XML files with a .topic extension that would be ran through an XSL transformation to give them the basic look and feel of the presentation style that was selected. However, using the basic transformations, the topics lacked several features present in the reference content such as collapsible sections and the language filter and they could not take advantage of build components to resolve links to reference and online content.
Starting with version 184.108.40.206, the help file builder supports conceptual content. This is similar in nature to additional content in that it allows you to add non-reference content to the help file that will appear in the table of contents. However, instead of HTML files or .topic files that contain HTML, conceptual content uses topic files that contain Microsoft Assistance Markup Language (MAML). This is basically XML conforming to a well defined schema that describes the structure of the conceptual content much like XML comments describe the structure of the code comments. There is no layout or style information within the MAML files. Instead, they are ran through a series of XSL transformations using Sandcastle's BuildAssembler tool similar to the reference (API) content so that they match it in appearance and features.
MAML is quite different than HTML but it does offer many of the same capabilities. The main difference is that the XSL transformations are responsible for applying the style and formatting, not the elements in the MAML topics. It does take some getting used to but the end result is well worth it. The look of the conceptual topics is much more consistent with that of the reference topics. As such, preference will be given to conceptual content when adding features to the help file builder. While it will continue to support the older additional content models, they will not receive any further enhancements.