From our FCOM:
The PSEU, its sensors, and its input signals are monitored for internal faults. When designated faults are detected, a PSEU light on the aft overhead panel illuminates, and the OVERHEAD system annunciator light and MASTER CAUTION lights illuminate. The PSEU light can be reset following a maintenance BITE check or repair of the cause of the fault.
So I would say a simple fault is any fault that hasn't been "designated" and if it is a "designated " fault the bite test or rectification are needed to reset it.