Modbus IO device naturally has a serial bus interface or an Ethernet interface on top of it. Both the serial bus and Ethernet bus have modbus as their high level protocol, however, because of the difference between the Ethernet and the serial bus, they are called modbus TCP bus and modbus RTU bus respectively after being given the modbus protocol. Modbus communication is characterized by the master tool will actively communicate with the slave tool, while the slave tool can only passively receive commands from the master and then give feedback.
Modbus remote IO device is as a slave and can only collect information from the outside world or send commands from the master, it cannot handle calculations for command issuance, those are the things of PLC or microcontroller.
The IO part, depending on the need, modbus IO devices may have digital or analog IOs, and digital and analog IOs can be divided into many categories. Just select according to the actual needs. If you want to collect the temperature signal of the steel mill, you will need to install an analog IO.