RPTRS ARE CLRED FOR AN APCH THAT IS UNAUTHORIZED FOR CAT 'D' ACFT.

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THIS EVENT INVOLVES A RAPIDLY CHANGING APCH ENVIRONMENT AT DFW INTL ARPT. WE HAD PREPARED AND BRIEFED A LNDG AT DFW USING AN ILS RWY 18R APCH. THE CAPT MENTIONED TO BE MENTALLY PREPARED FOR A POSSIBLE RWY CHANGE TO RWY 13R AND THE ILS RWY 13R APCH PLATE WAS REVIEWED BRIEFLY AND SILENTLY BY BOTH PLTS. I; THE FO; MADE A COMMENT ABOUT NOT BEING ABLE TO ACCEPT A 'CONVERGING ILS RWY 13R APCH' DUE TO OUR CAT 'D' STATUS. DURING THE LETDOWN AND APCH VECTORING STAGE OF THE FLT; APCH CTL GAVE US A HDG TO 'INTERCEPT THE LOC RWY 13R' FOR AN ILS RWY 13R APCH. A RADIO FREQ CHANGE WAS THEN GIVEN FOR US TO CONTACT THE FINAL CTLR WHILE WE WERE FLYING THE LOC FOR RWY 13R; RECONFIGURING THE ACFT; FMS; AND DOING APPROPRIATE CHKLISTS; DSNDING FOR GS INTERCEPT ALT; AND RETUNING THE RADIOS. THE FINAL APCH CTLR WAS EXTREMELY BUSY CONDUCTING ACFT TO BOTH RWY 18R AND RWY 13R. WE WERE INSTRUCTED TO KEEP OUR AIRSPD UP; CONTACT DFW TWR; AND CLRED FOR THE 'CONVERGING ILS RWY 13R APCH.' THE CAPT SWITCHED TO TWR FREQ AND AT THE SAME TIME I QUESTIONED HIM ON THE LEGALITY OF THE 'CONVERGING' APCH FOR CAT 'D' ACFT. THE RWY WAS NOW IN VISUAL SIGHT FROM THE COCKPIT AND TWR CLRED US FOR AN UNEVENTFUL LNDG ON RWY 13R. OUR DECISION TO LAND WAS MADE ON THE SIGHTING OF THE RWY; THE WX COOPERATED AT THE RIGHT MOMENT. THE QUESTIONS INVOLVED HERE CONCERN: 1) ATIS NOT ADVISING OF 'CONVERGING APCH.' 2) THE INITIAL APCH CTLR NOT ASSIGNING AN APCH. 3) EXTREME FREQ CONGESTION DUE TO NUMBERS OF ACFT HANDLED BY APCH CTL. 4) ABBREVIATED APCH BRIEFING DUE TO LAST MIN RWY CHANGE; AND 5) APCH CTL NOT KNOWING LIMITS OF CAT 'D' ACFT.