The best reason to create a Modular Help system is when you need to deliver a “plug and play” Help system that:

• Includes several separate Help files BUT appears to the end user as a single Help system.
• Looks seamless, even if Help files are excluded from the system. The table of contents and index simply omit the missing Help without displaying error messages.
• If the user installs an excluded Help file in the future, the Help will be added to modular system in the proper position.

The most common reason to create a Modular Help system is for a modular software system. That means your software is sold in separate modules or templates and you don’t know what the user will buy. A Modular Help system will work properly no matter what combination of options the user installs. The Help for each software module is installed with that module, while a parent (or hub) Help project that ties all of the Help files together  is installed with the core software system.

There are other reasons to create a Modular Help system, but they are not as compelling:

• Large documentation sets – If you wish, you can “chunk” information into several small Help systems and create a modular system rather than deploy one large Help system. If only a small percentage of your Help system ever changes, this may be a good option.
• Help systems with future Help projects planned – If you plan to release your Help system in stages, you can pre-position placeholders for future Help projects before they are released (for example, FAQs, shortcuts, etc.). Instead of distributing the entire Help system each time you add to the system, you only need to distribute additional Help files.
• Your documentation team shares responsibilities across one large project – Modular Help can be a solution for a large project with many authors. In this case, using Doc-To-Help’s Team Authoring or SharePoint integration may be a better solution.

With Doc-To-Help, you can create a Modular Help systems for three Targets (outputs): HTML Help (.chm files), NetHelp, and Microsoft Help Viewer 1.x (this output is only for use with MS Visual Studio 2010).

HTML Help is the best choice for a modular software system, because only a single .chm file needs to be installed for each software module.

In Part 2, we will talk about how to create a Modular Help Project, but in the meantime, you can take a look at the Doc-To-Help documentation: 

http://helpcentral.componentone.com/nethelp/d2h2011/WordDocuments/creatingamodularhelpsystem.htm