Merge Modules

These are a feature of Windows Installer that provides a standard method for delivering components, insuring that the correct version of a component is installed.
A merged module contains a component such as a .dll along with any related files, registry entries and setup logic.
These cannot be installed directly but are rather merged into an installer for each application that used that component.
This insures that the component is always installed consistently for all applications.
Each merge module contains unique version information that is used by Windows Installer to determine which applications use which component preventing premature removal of a component.
A new merge module should be created for every incremental version of the component. A merge module should never be updated after it has been included in an installer.

Merge Module Project

If you have more than one project that uses files in common, this option helps you make intermediate merge modules
You can then integrate these into your final deployment project.