Hi Soracha,
RECFA HRRFC is the RFC connection to the HR system. So if you use HR and E-Rec on different systems, you use that switch to map E-Rec with HR.
RECFA UI2BL is the switch for the distributed WebDynpro scenario, which means you have a UI system and a backend system. This switch has to be maintained only in the frontend system and includes the RFC connection to the backend system. This one has to be a trusted system connection.
->On the backend system you have to set-up RECFA BL2UI with the RFC connection to the frontend system and switch RECFA SYEMP defines the system where your employees are logging on.
It's also very important that the SICF services are configured as mentioned in the IMG step documentation -> IMG -> Technical Settings -> User Interfaces -> Candidate -> Front-End Candidate -> Specify E-Recruiting Services (Web Dynpro ABAP).
Regards,
Nicole