POTENTIAL CONFLICT LTSS WHEN ACR B727 DSNDS TOO CLOSE TO AN L1011 MAINTAINING 9000 FT.

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CLRED BY APCH TO DSND TO 7000 FT. AT ABOUT 13 DME; ATC CALLED COMPANY L1011 TFC AT 1 O'CLOCK; 4 MI. WE HAD THE TFC VISUALLY AND ON TCASII AND ACKNOWLEDGED VISUAL CONTACT TO ATC. THEN RECEIVED A TCASII TA ALERT ON THE L1011. AT 11 DME PASSING 9700 FT; WE WERE TOLD BY APCH THAT WE WERE ONLY CLRED TO 10000 FT. AT THE SAME TIME; RECEIVED A TCASII RA TO 'CLB; CLB.' THE PF (CAPT) IMMEDIATELY STOPPED DSCNT AND CLBED TO 10000 FT. WE CHKED OUR ALT BAROMETRIC SELECTOR AND IT WAS SET AT 7000 FT AND INFORMED APCH THAT WE HAD BEEN CLRED TO 7000 FT. APCH SAID THAT WAS FOR ANOTHER ACFT. OUR DSCNT WAS STOPPED AT 9600 FT (600 FT ABOVE L1011) BEFORE CLBING AND WE PASSED ABOUT 1/2 - 1 MI BEHIND THE L1011 WHILE KEEPING HIM IN SIGHT THE ENTIRE TIME. AFTER LNDG THE CAPT CALLED THE APCH CTL SUPVR ON THE PHONE AND THE SUPVR ACKNOWLEDGED THAT THE TAPES CONFIRMED WE HAD IN FACT BEEN CLRED TO 7000 FT; NOT 10000 FT; AND IT WAS THEIR ERROR. LUCKILY WE WERE VMC AND NOT IN IMC. I READ BACK ALL ALT CLRNCS AND ALWAYS USED OUR FULL CALL SIGN; IE; 'ACR X CLRED TO 7000 FT.'