Modsak Manual v3.14 - 2.3.4. Special registers
In addition to the four main address ranges, there are two special registers that have their own Modbus commands, and are therefore mapped separately. The Exception Status Register is used by the 'Read Exception Status' command (function code 7), and the Diagnostic Register is used by the 'Diagnostics - Return Diagnostic Register' command (function code 8, sub-function 2). You only need to configure these mappings if you are using Modsak in slave mode and your master uses these commands.
You should enter in the Exception Status Register or Diagnostic Register field the model address of the register to be used. This register may be one that is also used in one of the standard address ranges (i.e. it may be overlaid), or it may be a dedicated register. You may also use a group of 1-bit registers as the exception status or diagnostic register - in this case you should enter the model address of the first register in the group. Enter "-1" to disable the register.