Execption handling in RFCs

Hello All.

I've read about the "Two Pre-Defined Execptions" for a RFC.

a> SYSTEM_FAILURE

b> COMMUNICATION_FAILURE

AND that, while calling the RFC we can code:

EXCEPTIONS

SYSTEM_FAILURE = 1 MESSAGE msg

COMMUNICATION_FAILURE = 2 MESSAGE msg

Can anybody please help me on this? Both concept wise and the above code. I mean, what do you mean by Pre-Defined and how to successfully use it.

Thank You.