Devices Tested: Opto 22 Groov Epic PR 1 (3.5.17.10) and Codesys Control Win V3 x64 (3.5.18.0)
Codesys Version: 3.5 SP18 64bit
Configuration:
Codesys is acting as a Modbus Slave (ModbusTCP Slave Device).
General Configured Parameters:
Holding Registers: 500 (%QW) Writeable
Input Registers: 2 (%QW)
Coils: 200 (%QX)
Discrete Inputs: 0
Data Model: 0/0/0/0
[-img src=2022-07-21 13_21_42-Window.png width=50%: missing =-]

Modbus Configurator automatically configured variables visible in the I/O Mapping section. Holding Registers beginning at %QW14. Input Registers beginning at %QW514
[-img src=2022-07-21 13_22_15-Window.png width=50%: missing =-]

I am unable to discern what the "Holding-and input register data areas overlay" checkbox does. Contrary to the help documentation, linked below, I can write to the holding registers both from within the application and externally through Modbus regardless of the configuration of this option.

Does anyone have a good understanding of what this option does?

https://content.helpme-codesys.com/en/CODESYS%20Modbus/_mod_edt_slave_device_tcp.html