Hello,
RCCU is hardware state machine which compares input signals and in case of mismatch it reports fault to FCCU unit.
Usually RCCU faults are present after power on reset or due to debug mode where RCCUs are disabled.
You can inject FCCU RCCU fault by FCCU NCFF register.
There is no specific documentation on RCCU as it is not user configurable.
Best regards,
Peter