It is a Mach3 configuration file. Every Mach3 profile stores its settings in an XML file located in the Mach3 installation directory (C:\Mach3, by default). If Mach3 is loaded using the “MyMill” profile, those settings would be stored in MyMill.xml. The name of the currently loaded profile can be seen in the lower-right corner of the Program Run screen in the default screen set. See “What is a profile?”