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.