A modular help system is a help system that consists of multiple Help+Manual projects that can be published as a single project. This can make it easier to manage large projects.
In addition to this, it may sometimes be practical to break up projects into modules when you are working in a team, even though it is also possible for all team members to work in the same project together in Help+Manual Professional. For example, individual team members may want to work on their parts of the project on their laptops.
See Modular Projects in the Reference section for some more background information on modular help systems.
Modular help systems
Modular Projects (Reference)