ModMaster Manual v3.11 - 4. File Registers
File registers are used to store the values read and written by the Read File Record (function code 20) and Write File Record (function code 21) commands.
You can send Read File Record and Write File Record commands using the Define Command page, without needing to define any file registers. Alternatively, you can define some file registers, as described in the following sections, and use the File Register Table buttons to request file register reads and writes.
Note that file registers are referred to by several different names in the Modbus specifications. They are variously called "file records", "registers" and "references" in the current Modbus specification, and they are called "General References" or "Extended Memory file (6x) references" in the original Modbus specification and the Modbus/TCP specification (with the corresponding commands being called "Read General Reference" and "Write General Reference").