Laser GCode P parameter

I’m just looking through the GCode generated by Luban for the laser module, and there’s a strange parameter in the M3 command.

G90
G21
; G-code START <<<
M106 P0 S255
G0 F1500
G1 F500
G0 X-42.35 Y-29.56
M3 P100 S255
G1 X-42.06 Y-29.56
M5
G0 X-41.78 Y-29.56
M3

It’s all good until the first M3 command. M3 starts the spindle (turns the laser on). I can’t find any reference to the P parameter. S (S255 in this case) is used to set the spindle speed in standard GCode, and it’s used here to set the PWM period for the laser. in this case, it’s set to 255, which means the laser is at full power. The P parameter appears to be a reference to power level (0%-100%), but I can’t find any reference to it anywhere.