FLC OF AN SMT ACR ACFT STARTED AN IFR APCH OUTSIDE THE OM WHEN THE LATEST WX RPT INDICATED THAT THE WX WAS BELOW LNDG MINIMUMS.

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WHILE BEING VECTORED FOR THE ILS 23R APCH AT TYS; 5 MI OUTSIDE THE OM; APCH ADVISED THAT RVR WAS 1200. COMPANY OP SPECS ALLOW CREW ONLY TO OPERATE ACFT TO 1800 RVR. WHEN CTLR HANDED US OVER TO TWR; HE ADVISED US THAT RVR WAS 1200. WE DECLARED A MISSED APCH AND REQUESTED VECTORS FOR ANOTHER TRY. THE TWR ADVISED US THAT RVR WAS VARIABLE AND CONDITIONS WERE IMPROVING FAST. ONCE WE WERE WITH APCH AGAIN AND VECTORED JUST OUTSIDE THE MARKER FOR THE SAME APCH THE CTLR ADVISED US OF THE SAME RVR. AFTER HANDING US OVER; THE TWR ADVISED RVR 2000 VARIABLE AS WE UNDERSTOOD IT. WE CONTINUED THE APCH AND FOUND THAT THE RVR WAS INDEED VARIABLE AS THE FOG WAS PATCHY AND SCATTERED AND THE RWY WAS VISIBLE FROM THE OM INBOUND EASILY. AN UNEVENTFUL; SAFE LNDG WAS MADE. AFTER LNDG THE COPLT QUESTIONED THE MINIMUMS AND RAISED A QUESTION IF WE WERE INDEED LEGAL; AS THERE WAS SOME CONFUSION AS TO WHAT THE TWR CALLED THE RVR AND AS TO WHAT WE UNDERSTOOD. IN FUTURE OPS WE WILL ENDEAVOR TO CLARIFY ALL RPTS OF RVR CLRLY. FACTORS AFFECTING THE FLT HAD BEEN SOME SLIGHT FATIGUE AS WE HAD BEEN CALLED AT 0330 AM FOR THIS FLT. IN THE FUTURE; ANYTIME RVR IS BELOW LNDG MINIMUMS (INCLUDING VISIBILITY); IT WOULD BE HELPFUL IF ATC WOULD STATE THIS BEFORE ISSUING ANY CLRNCS TO EXECUTE AN APCH TO ASSURE THAT BOTH PARTIES ARE INDEED AWARE.