Hi Siva krishna,
Extending further as Caique replied to your message . RFCUSER appears in transaction change history in the scenario when the changes were made by some one in another SAP system and that flowed down to CRM or ECC . In that case if you have set the RFC settings of that SAP system under CRM or ECC as a fixed user : RFCUSER then the user under CRM or ECC change history will appear as RFCUSER.
If your scenario is not regarding the changes done in a remote system and being moved to CRM or ECC , then I would request you to kindly explain your scenario.
/Hasan