Variables are the interface between the device driver and the other components of the software, specially the user interface. Each variable acts as an end point for a parameter message. It may receive messages - which causes the driver to set this parameter at the physical device - but also may send it's parameter message in order to tell the user interface about the actual setting of this parameter.
Before you start writing the device driver, you should plan a list of variables which build the software interface to the device. Designing this interface should be done careful, you can save a lot of effort if you are able to re-use variable definitions from other devices. This not only saves the time to write the definitions, it also enables you to use the parameter screens available for this device in the sat-nms library.
If you intend to write are driver for a device which implements similar functions as an existing driver, it is recommended to copy the variable list from this device.