Hey RFI,
you can use nut for triggering events in case the UPS reports mains electricity failing (which would be easy to install on (Raspberry Pi 3-based-) Onefinity Controller since it’s in raspberrypi.org repository). Such an event would be to pause the program, and for stopping spindle you may trigger the programmed Input Terminals available on any VFD. See also here.
And finally nut is usually used to shutdown the computer (in this case, the CNC Controller) just before the UPS’s batteries become empty.