CAPT OF AN LGT CONTINUED AN INST APCH AFTER RECEIVING BELOW MINIMUMS RPT PRIOR TO THE FINAL APCH FIX.

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WHILE ON APCH TO THE MONTREAL ARPT; THE TWR VISIBILITY AND RVR FLUCTUATED WIDELY DUE TO AREAS OF THICK FOG MOVING ACROSS THE ARPT. AFTER INITIALLY BEING CLRED FOR THE LOC/BACK COURSE TO RWY 6R; THE VISIBILITY DROPPED BELOW THOSE MINIMUMS. HOWEVER; THE RVR FOR RWY 6L IMPROVED FROM 1000 FT TO 6000 FT; AND AN ILS APCH WAS APPROVED TO RWY 6L (WHICH HAS APCH MINIMUMS OF 2600 FT RVR). AFTER BEING CLRED FOR THE ILS TO RWY 6L; ACFT WAS ON THE LOC AND DSNDING ON THE GS; AT APPROX 2000 FT MSL; TWR ADVISED THAT THE RWY 6L RVR WAS NOW 1800 FT. SINCE WE WERE ON THE FINAL APCH SEGMENT (ACFT DSNDING ON GS; BELOW THE GS INTERCEPT ALT OF 2500 FT) THE APCH WAS CONTINUED. AT DECISION HT THE RWY WAS VISIBLE AND THE LNDG WAS MADE. LATER I REALIZED THAT THE FINAL APCH SEGMENT FOR CANADIAN ARPTS BEGINS AT THE FINAL APCH FIX; IN THIS CASE AT 'BIRPO.' THE APCH SHOULD HAVE BEEN DISCONTINUED SINCE THE BELOW MINIMUMS RPT WAS RECEIVED PRIOR TO BIRPO. WITH THE LARGE NUMBER OF XBOARDED FLTS BTWN THE UNITED STATES AND CANADA; THERE SHOULD BE COMMONALITY OF ALL APCH DEFINITIONS AND TERMS. MY SIT FELL BTWN THE UNITED STATES DEFINITION (2500 FT MSL) AND THE CANADIAN DEFINITION (BIRPO FIX) OF THE FINAL APCH SEGMENT. THE FINAL APCH OF AN ILS SHOULD BE FREE FROM THESE CONFLICTING DEFINITIONS.