DIVERSION TO ALTERNATE DURING A NIGHT OP AFTER AN SF340 FLT CREW MAKES 2 ATTEMPTS FOR AN ILS RWY 23; QUESTIONING THEMSELVES IF THE APCH WAS LEGAL BASED UPON THE RVR GIVEN BY TWR AT CRW; WV.

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WE FLEW TO CRW; CHARLESTON; WV. WE COULD SEE THE ARPT ABOUT 15 MI AWAY; BUT THE ATIS WAS RPTING LOW VISIBILITY DUE TO GND FOG. WE GOT AN ORAL RPT FROM THE TWR WHO GAVE US A CURRENT PREVAILING VISIBILITY THAT WAS LEGAL TO START THE APCH. AFTER 2 UNSUCCESSFUL APCHS; WE DIVERTED TO OUR ALTERNATE; ROANOKE. A WK LATER; SOMEONE QUESTIONED IF WE WERE LEGAL TO START THE APCH. OUR FAA APPROVED FLT OPS POLICIES AND PROCS SAYS 'AN ORAL RPT FROM THE TWR CONTAINS A VISIBILITY VALUE SPECIFIED AS RWY VISIBILITY OR RWY VISUAL RANGE FOR A PARTICULAR RWY AT AN ARPT; THAT SPECIFIED VALUE IS CTLING FOR TKOFS; LNDGS.' BASED ON THIS STATEMENT; I BELIEVED I WAS LEGAL BECAUSE IT SAYS EITHER RWY VISIBILITY OR RVR CAN BE CTLING AND DOES NOT SPECIFY WHICH IS MORE RESTRICTIVE IF BOTH PREVAILING VISIBILITY AND RVR ARE RPTED. HOWEVER; AFTER CHKING WITH OUR CHIEF PLT FOR CLARIFICATION; HE SAID RVR IS ALWAYS CTLING; REGARDLESS WHETHER IT IS MORE OR LESS RESTRICTIVE THAN RPTED PREVAILING VISIBILITY. I REMEMBER THE CTLR RPTED BOTH RWY VISIBILITY AND RVR. I DO NOT REMEMBER WHAT HIS RVR RPT WAS. I REMEMBER HIS RWY VISIBILITY WAS LEGAL AND; BASED ON OUR MANUAL; I THOUGHT THE APCH WAS LEGAL. HOWEVER; IT MAY NOT HAVE BEEN LEGAL BASED ON THE RVR RPT. THIS PROB AROSE DUE TO INADEQUATE GUIDANCE IN OUR OPS MANUAL (IT ONLY SPECIFIES WHEN YOU HAVE 1 RPT OR THE OTHER; BUT NOT WHEN YOU'RE GIVEN BOTH). ADDITIONALLY; IF WE HAD DISCUSSED AND/OR ROLE PLAYED THIS SCENARIO IN OUR AIRLINE'S GND SCHOOL; THE AMBIGUITY WOULD HAVE BEEN CLARIFIED. SUPPLEMENTAL INFO FROM ACN 618308: AFTER REVIEWING THE SIT; I REALIZED THERE MAY HAVE BEEN A DISCREPANCY BTWN ATIS AND TWR RPTED RVR.