Each device intended to communicate using Modbus is given a unique address.
A Modbus command contains the Modbus address of the device it is intended for. Only the intended device will act on the command, even though other devices might receive it (an exception is specific broadcast-able commands sent to node 0 which are acted upon but not acknowledged).
All Modbus commands contain checking information, ensuring that a command arrives undamaged.
The basic Modbus commands can instruct a Remote Terminal Unit to change a value in one of its registers, control or read an I/O port; as well as commanding the device to send back one or more values contained in its registers.