At the center of the thermostat is a wax filled plug and a pin coming out one side (engine side). As the water heats up from the engine, it melts the wax, expands it and pushes on the pin to begin to open the valve. The wax is tuned to begin to melt at a certain temp and fully melt/expand above a slightly higher temp. It could start to open at 40C and be fully open above 49C. This would allow engine to warm more quickly on cooler days, but maintain max temps on warmer days.
If you are not sure what the temp rating is for a given thermostat, you can test it. Boil some water. In a separate glass vessel, submerge the thermostat in room temperature water. Measure the temp of the water thermostat is in. Slowly add the boiling water while watching the thermostat valve. As the valve begins to move, note the temp at that point. Continue to add boiling water until the thermostat is completely open. Note that temp for your overall working range.
Warning, A thermostat that has been operated too high above its working range can alter the working range. Meaning, if it gets too hot, it may not begin to open until a higher temp than before. So if you ever have an incident of over heating, it is imperative to test the thermostat range again or just simply replace it.