It is a Mach4 configuration file. Every Mach4 profile stores its settings in a Machine.ini file located in the Mach4 profiles directory (C:\Mach4\Profiles, by default). If Mach4 is loaded using the “MyMill” profile, those settings would be stored in C:\Mach4\Profiles\MyMill\Machine.ini. See “What is a profile?”