Please see the "options for commands" section of this manual for general information about the use of macros for configuration.
This lets you control which interface of "Msm.Merge?" ("MergeMod.DLL") you want MAKEMSI to use.
You list the versions you wish to use separated by semicolons and double quoted. So to support versions "Msm.Merge2.1" and fallback to "Msm.Merge2" you would supply "2.1;2" (with the double quotes!).