<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def zone(self) -> Optional[str]: """Zone the device is assigned to."""

        if self._device_category == DC_BASEUNIT:
            return None
        return '{:02x}-{:02x}'.format(self._group_number, self._unit_number)