Using function code 2, Modbus can read the discrete inputs from a Modbus slave, which is commonly used by SCADA and DCS field devices. Discrete inputs represent binary (i.e boolean) values that often map to switches, relays, or other sensors. A sample of discrete inputs read from the device are provided by the plugin output. The ability to read discrete inputs may help an attacker profile a system. (Source: Tenable Nessus)
As this issue is assessed as medium severity, users may evaluate their environment and schedule the update in the next maintenance or update cycle.
This alert has come from: https://www.moxa.com/en/support/product-support/security-advisory/mpsa-257317-security-enhancement-modbus-tcp-discrete-input-access