M1 Optional Stop

M1- (M&S)Optional stop. Main and Sub

If the OPTIONAL STOP button is on (lit) the executing line will
stop until the CYCLE START button is pressed again, then operation in
that Program starts running again. If the OPTIONAL STOP button is not lit
the M01 command is skipped. This may shutoff coolant. use M8,M13,M14 to
restart spindle and coolant