If you have an individual boiler in the basement and a room thermostat which is wired you do not need the Extension Kit. However if you have an individual boiler with a wireless thermostat, or no thermostat, you need the Extension Kit.
The Extension Kit consists of the Extension Box, a cable to connect it to the boiler and plugs for different boilers. The Extension Box is connected to your boiler via cable and placed next to it. The Smart Thermostat and the Extension Box are wirelessly connected.