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Machine variables are similar to player variables, except that machine variables are machine-wide and persist between games. (In fact, machine variables can be configured to be saved to disk so they also persist between reboots of MPF.)
Like player variables, you can use machine variables in your config files, particularly in text display widgets, to show things on your display.
You can create your own machine variables in your configs. There are also several machine variables that are automatically created. Here’s a list of the machine variables that are “built in” and available for use in your configs: