In the Modbus protocol, the function code is used to tell the slave device what kind of action to perform, such as reading or writing data.
Modbus function codes specify the type of operation to be performed on the registers. For example, function code 03 is used to read holding registers, and function code 06 is used to write a single register.
Each function code is a single byte in size and is positioned at the start of the PDU (Protocol Data Unit) in the Modbus message structure, directly influencing how the slave interprets and executes the request.
"The Modbus Protocol Explained," by Schneider Electric.
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