FLC OF AN MLG ACR ACFT WAS DSNDED TO THE WRONG ALT RESULTING IN LTSS.

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WE WERE INBOUND TO CLT FLYING THE CHESTERFIELD 7 ARR (CTF 7-CLT) LEVEL AT 11000 FT. CLT APCH ISSUED US A CLRNC TO 'SLOW TO 210 KTS AND AT PLT'S DISCRETION MAINTAIN 6000 FT.' I REPLIED; 'AT CAPT'S DISCRETION MAINTAIN 6000 FT SLOWING TO 210 KTS (CALL SIGN). WE WERE TOLD WE WERE #3 FOR THE APCH TO RWY 23. SEVERAL MORE VECTORS WERE ISSUED DURING OUR DSCNT. WHEN LEVEL AT 6000 FT WE VISUALLY AND ON THE TCASII SPOTTED AN MLG AT OUR 11 O'CLOCK POS IN A TIGHT L TURN TOWARD US. THIS ACFT LEVELED AT 6000 FT FROM HIS CLBOUT. THIS WAS CONFIRMED BY OUR TCASII. THE MLG THEN MADE A R TURN TO PASS BEHIND US 1-2 MI. NO TCASII ADVISORY TO MANEUVER WAS ISSUED BY OUR UNIT. I QUESTIONED APCH CTLR ABOUT THE CLOSE PROX TFC. THE FREQ HAD BEEN VERY BUSY BUT WAS NOW VERY QUIET. THERE WAS NO REPLY TO MY TFC QUESTION BY THE CTLR. WE WERE THEN ISSUED ANOTHER VECTOR AND HANDED OFF TO ANOTHER FREQ. THE REST OF THE APCH TO LNDG WAS ROUTINE. ONLY AFTER ARR AT THE GATE WAS THE CAPT ASKED TO CALL THE LCL ATC MGR. DISCUSSION ON THE PHONE WITH THE ATC MGR INITIALLY STATED WE HAD BUSTED AN ALT CLRNC. HE SAID THE CLRNC WAS ONLY TO 9000 FT. WE DISPUTED HIS THEORY AND ASKED IF IT WERE POSSIBLE TO DO A QUICK REVIEW OF THE TAPES. HE STATED THAT TAPE REVIEW WAS POSSIBLE AND TO CALL HIM BACK IN 30 MINS. THE CAPT RETURNED TO CALL AND WAS TOLD THAT FURTHER REVIEW OF 'THE TAPES' INDEED DID DISCLOSE THAT OUR ACTIONS INFLT HAD BEEN CORRECT AND THAT THE ATC CTLR HAD ISSUED US THE WRONG ALT. WE HAD DSNDED TO 6000 FT PROPERLY IN RESPONSE TO THE INCORRECTLY ISSUED CLRNC. CTLR WORKLOAD; FREQ CONGESTION; READBACK/HEARBACK -- WHAT'S NEW?