That is a rpm gauge mate. It connects to plug and calculates how often it sparks it doesn't limit anything.
It come with a brass adapter ? put that in where your themostat goes. I'm not sure how to wire revs in :/
The thermostat goes in the thermostat hole I havent used the Stage6 meters but its multifunction (rpm, head temp and engine hours i think) so should have atleast 3 or 4 different things that need connecting up. RPM meters normally have a coil of wire that wraps around the plug itself that picks up engine pulses. I havent used this particular one but I dont reckon they'll be all that different