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A merge module is a special kind of Windows Installer database that
contains the components needed to install a discrete software bundle. A
merge module cannot be installed alone, but must be merged into a standard
Windows Installer installation during compile of the installation.
Typically, a merge module or a collection of merge modules related by
dependencies installs a software product or portion of a product at runtime.
The purpose of merge modules is to let you add self-contained software
modules to multiple installations.