CPR JET GIVEN A DSCNT THAT THEY KNEW WOULD TAKE THEM THROUGH OCCUPIED ALT. TFC WAS DETECTED ON THEIR TCASI AHEAD AND THEY NEVER QUESTIONED THE ALT ASSIGNMENT UNTIL CTLR ASKED WHAT ALT THEY WERE GOING TO.

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TFC WAS ISSUED TO OUR ACFT WHILE WE WERE DSNDING TO FL220. TFC WAS AT 1 O'CLOCK AND FL210. WHILE PASSING THROUGH FL230 THE CTLR SAID 'TFC 1 O'CLOCK (SO MANY MI) FL210. MAINTAIN FL204.' I RESPONDED 'ROGER DSND AND MAINTAIN FL200.' I ASSUMED WE HAD LATERAL SEPARATION. AFTER PASSING THROUGH FL220 I PICKED UP THE TFC VISUALLY AND THEN BEGAN TO NOTICE IT TO START TO RISE ABOVE THE HORIZON; REALIZING WE WERE GOING THROUGH HIS ALT. AT FL210 THE CTLR ASKED US TO VERIFY WE WERE DSNDING TO FL220 AND I RESPONDED 'NEGATIVE WE WERE GIVEN FL200.' AT ABOUT THAT TIME OUR TCASI SOUNDED 'TFC; TFC.' NO EVASIVE ACTION WAS NEEDED AS WE WERE WELL SEPARATED VISUALLY. SUPPLEMENTAL INFO FROM ACN 363580: PASSING THROUGH ABOUT FL210; THE CTLR ASKED US TO CONFIRM OUR DSCNT TO FL220. COPLT REPLIED -- 'NEGATIVE; WE WERE CLRED TO FL200.' CTLR ADVISED US OF TFC WHICH WE HAD IN SIGHT AND HAD BEEN WATCHING FOR SEVERAL MI. NO FURTHER COMMENTS WERE MADE. OBVIOUSLY THE CTLR DID NOT INTEND TO CLR US TO FL200; BUT BOTH MYSELF AND THE COPLT CLRLY HEARD HER SAY DSND TO FL200. I WOULD NORMALLY HAVE CAUGHT THE CTLR'S MISTAKE EXCEPT I WAS PREOCCUPIED WITH LOADING AN APCH IN THE NAV MGMNT SYS AND WASN'T WATCHING THE TCASI (TCASI SYS).