Also referred to as "merge package file." A merge module (.msm file) is a single package that includes all files, resources, registry entries, and setup logic to install a shared component. Merge modules include all the information necessary for a Microsoft® Windows® installer to install a shared component.
Merge modules are consumed by installer package (.msi) files. Microsoft® Visual Studio® Installer facilitates both creating .msm files and consuming them in an .msi file. You can distribute your shared components in merge modules, and you can use shared components by including the appropriate .msm files in your .msi file.