They are the wire on/off switches, borrowed from spindle and coolant codes. M3 (spindle on) or M8 (coolant/flood on) is repurposed to energise the hot wireet M5/M9 turn it off. Which pair you use depends on how your controller's output is wired — FluidNC lets you map the wire to the spindle or an accessory output.
With a PWM heat control, an S value alongside M3 sets the duty cycle, i.e. the temperature. https://cncfoam.com places the wire-on near the start (with a pre-heat dwell) and wire-off at the end, so the foam is only ever cut by a properly heated wire and the heat is killed when the job finishes.
With a PWM heat control, an S value alongside M3 sets the duty cycle, i.e. the temperature. https://cncfoam.com places the wire-on near the start (with a pre-heat dwell) and wire-off at the end, so the foam is only ever cut by a properly heated wire and the heat is killed when the job finishes.