Question
What is the M terminal on the connection board inside the control box

Answer
Hey Rory, this would be for a master valve hook up ( if you have one), not all systems use a master valve. Or it could be hooked up to a pump start relay (if you have a pump) not all systems have a pump. If you don't have a master valve, or a pump start relay, then it is not used. Good luck and have a great day.