I just got some good info on this from one of our engineers. The Unit ID/Slave ID for TCP/IP is usually 1 or 0. Unit ID is not needed since the PLC is identified by IP Address but Modbus devices using TCP/IP should have a UnitID incase they convert TCP/IP to Serial\RTU, in which case the Unit ID would be needed .
Here is also a good thread on this topic: https://modbus.control.com/thread/1026171903