The mailbox registers (5580 -5679) are used to store any user data values within UINT16 (0-65335) size by write command through Modbus application.The written user data values can be readable by using the same registers.Any Modbus application can be used for this read and write function.
So basically those registers are for keeping and referring any downstream meters values by writing in this registers.
So basically those registers are for keeping and referring any downstream meters values by writing in this registers.