TWIN BEECH 18 SHOT AN APCH AND LANDED WHEN THE PREVAILING VISIBILITY WAS ONE HALF MI AND THE RVV WAS GIVEN AS 5 SIXTEENTHS TO 7 SIXTEENTHS. THE MINIMUMS ARE ONE HALF MI. AFTER A DILIGENT SEARCH; RPTR WAS UNABLE TO DETERMINE; FOR SURE; WHETHER OR NOT RVV IS CTLING.

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I DEPARTED BFI AT XA20 WITH A LITTLE LESS THAN AN HR ENRTE TO MOSES LAKE; WA. I WAS ON A DOWNWIND LEG FOR THE ILS RWY 32R. THE ATIS WAS CALLING THE VISIBILITY A HALF MI IN FOG. I DON'T RECALL THE REST OF THE WX. CONTACTING APCH CTL; I WAS AGAIN ADVISED 'VISIBILITY HALF MI.' WHEN THE CTLR GAVE ME THE BASE LEG VECTOR; HE ADVISED ME 'RVV THREE SIXTEENTHS - FIVE SIXTEENTHS; PREVAILING VISIBILITY ONE HALF MI.' I DO KNOW RVV STANDS FOR RWY VISUAL VALUE; AND FROM WHAT I CAN GATHER; IT IS BASICALLY THE SAME THING AS RVR BUT GIVEN IN FRACTIONS OF MI. AT A VERY BUSY POINT ON THE APCH I WAS ADVISED OF SOMETHING I WASN'T SURE WHAT IT MEANT. CAN I LEGALLY SHOOT THE APCH UNDER PART 135? WHAT IS CTLING RVV OR PREVAILING VISIBILITY? I DID HAVE TIME TO GLANCE DOWN AT THE APCH PLATE TO SEE IF RVV WAS MENTIONED ON THE MINIMUMS SECTION; IT WAS NOT. I WAS ALSO TALKING TO OTHER FREIGHT PLTS ON A COMPANY FREQ WHO ALSO WEREN'T SURE WHAT WAS CTLING; ONE OF WHOM SHOT THE APCH BEHIND ME. I CONTINUED THE APCH AND LANDED WITH THE REQUIRED HALF MI VISIBILITY.