The switch can work either as a thermostat or to set a constant heat output.
There are two lights - a green one always stays on when the unit is running (you can basically ignore that one)
The other light can be either red or green, you press the black button to switch between modes.
Red mode: Thermostat control. The heater will switch on and off at set temperature levels But note that switching on and off is a fairly slow process as it has to burn out remaining fuel in the system and go through a 'controlled shut down' and the reverse at start up.... so dont expect it instantly switch on and off!
Green mode: Constant output. The heater will run constantly at a constant level of heat output. Turn the knob up all the way and you get very hot air coming out at 2kw output. Turn it half way and you get 1kw of heat output constantly, turn on very slightly and you get a low level of heat constantly.
Operating instructions for the SIM controller are in this article.