The different types of plinth heater refers to the actual heating component source . The Hydronic heaters generate heat from the central heating system through a copper- aluminium heat exchanger. Similar to your car heater it is a radiator with a fan, the heater is connected to the central heating pipes over which the electric fan blows air, warming it before blowing it into the room.
The sole electric types are in essence plinth mounted electric fan heaters. The sole electric models should now only be supplied with the thermostat and set back timer to comply with Lot 20 regulations.
This was put into place that all heaters will have to incorporate intelligent room temperature controls to minimise wasted energy. The wireless electric plinth heaters all have a setback timer and wall mounted wireless thermostat.