Once the initial track plan and track bus have been designed usually the next question is how do I control all these switches? The answer is: Accessory Decoders. They can also be called switch decoders or stationary decoders. In the simplest form they have two wires to the track, two wires to the switch machine. Power for the accessory decoder (and the switch it is controlling) is provided by the track. This gives you control of your switches from an NCE cab. Accessory decoders are based on the type and manufacturer of the switches being used.
Note that this is not a requirement. If you have manual thrown switches or switches that came with a separate controller, they can be used independently alongside your DCC system.