Hello Papiya Chatterjee,
The primary purpose of this table GRFNREPFIELDMAP is to maintain Special fields for reporting.
Using this table, its now possible to change the data element of reporting field without changing the DDIC structure. The data element can be changed in the table view GRFNREPFIELDMAP.
If you go through this note1710049 - Drivers and impacts are missing in the reporting, you will understand how a new content is added to the table GRFNREPFIELDMAP.
Let me know if you are looking for further details.
Regards,
Rakesh Ram M