What is this type of hot plate and how do I use it?

I managed to locate the cooktop instructions. The hotplate is one with a thermostat control and the raised section is the temperature sensor.

From the instructions:

The right rear hotplate control is a thermostat. The control may be set anywhere between low and high but must be turned anti-clockwise back to off.

This control switches the hotplate on and off to maintain the set temperature.

I used to have an electric oven with similar hotplates and while I can't find a reference because it was such an old model I recall it was a safety cut-off. It would simply stop supplying power to the hotplate once a pot was removed, something you could easily confirm by seeing what happens if you turn on one of the hotplates without anything sitting on top.

As for the question of usage electric hotplates often don't use a thermostat, they use something commonly referred to as a "simmerstat". They simply control the duty cycle of the heating element, so in your example setting of three it would spend approximately 30% of the time on and 70% off.

I've tended to spend most of my life cooking with them and can't really think of shortcuts to learning how to use one effectively other than experimentation and getting used to them. For example I was just cooking some dried chickpeas and while I started things off at around 7/10 to bring them to the boil I later dropped that to around 2/10 and as liquid evaporated it ended up spending most of the time at 1/10 and that was still a reasonably rapid simmer for a small pot.

Centre unit is a contact thermostat which measures the heat of the pan and also its contents. The graduations on the control knob are for you to set desired temperature of that thermostat, from hand hot (keeping something warm) to boiling vigorously.

Try putting a pan of water on the hotplate, then turn the knob to number 1. The hot plate will go to maximum briefly and then reduce to a low temperature, and then just turn on/off as required to maintain the temperature you set. Because the element is switching off/on every few seconds it won't look hot, but it will be. The central sensor is measuring the temperature of the pan (and therefore the temperature of the water you put in the pan) and is switching the element on and off to keep the water at the temperature you have selected. It is ideal if you are heating milk and want it to be hot but not boiling. Set the knob to about number 2, but you will have to experiment to get the right temperature. Might be 1.5 to 2.5 (a very cold kitchen will need a slightly higher setting). Then you won't have the milk boil over on you.

Good for pressure cookers too because you can set the thermostat, by experiment, to about number 3 or 4. Heating element will glow red hot until pan reaches the set temperature and will then maintain that pan temperature for you. For a pressure cooker you want to set the temperature so that the steam pressure in the cooker is just lifting the pressure weight you set on the pressure cooker. Adjust the knob to achieve this. If you set the temperature too high you will have steam blasting out of the pressure cooker, which is an expensive waste of electricity and money.

Simmerstat control enables you to cook using minimum electricity, but you need to get used to it to make it work effectively. Remember that the control knob sets the temperature you want the pan (and its contents) to reach. You don't control the heating element, the thermostat does that for you. Always ensure the pan is centrally placed over the thermostat. If you pull the pan to one side of the ring, off the thermostat, then the thermostat will turn the element up to maximum because, without the heat from the pan, the thermostat will feel cold and turn the heat up. Always turn the knob to off before you remove the pan. That ring, used correctly, can save you a lot of money. Good luck.

The centre piece is a thermostat connected to the control knob of the cooking plate. It is from quit an old cooking top.