The Salus EP200 has two channels, one for the Central Heating and one for the Hot Water. The electrical current capacity of the Salus is limited to 8A, so it should be wired to a relay or contactor suitable for switching the higher currents associated with electric heating.
The times that you want the Central Heating and Hot Water to be 'on' for can be programmed into the Salus. If you don't have operating instructions for it, you can find them by clicking here.
Much of how it actually works will depend upon how the installer has wired it up. Normally, I would expect the Hot Water settings to control the immersion heater and the Central Heating settings the radiators. Usually the heaters and immersion would be left permanently switched 'on' at their local isolators and switches, and the Salus would switch the power to them 'on' and 'off' according to the settings programmed into it.
If you can't fathom it out you should ask the letting agency / landlord to show you how it works.