Grbl only has one Spindle output so that would be an either/or situation for spindle control Unless you want to use the IOT relay for something else, then use the Coolant output (M8/M9 to turn it on/off)
Actually I wanted to use the IOT relay for the spindle water pump so it sounds like I can just connect the IOT to the coolant output?
Thanks for the extra details, it helps to have it Yes, according to the datasheet (attached), the IOT Relay can use a control signal of 3-48v DC (so the 24v Coolant output should do nicely) Haven't tested if personally, but please let me know if you have any issues