In and HTML Help (CHM) you can create genuinely modular help systems, in which your output consists of multiple help files that are all displayed in a single Table of Contents (TOC). This makes it possible to create different versions of your help simply by including or excluding help file modules from your distribution package: Modules that are not there are excluded from the TOC automatically.
In modular HTML Help (CHM) projects in which the child projects are merged in runtime mode, the master project must contain at least one topic and the default topic must be set in the master project. If you don't do this you will get an error opening the main CHM and the indexes of the child CHMs won't be available.
You can also use Help+Manual's modular project features in other output formats. However, here the advantages of using modular projects is only available during the editing stage. The output is always a single integrated help system. It is effectively the same as publishing your help from one large project.