RPTED LOSS OF SEPARATION WHEN FLC OF ACR B737 MISCOPIES DSCNT ALT AND CONFLICTS WITH UNKNOWN TFC AT 11000 FT.

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AS WE DSNDED ON THE STAR N OF BSR WE WERE GIVEN A VECTOR BY ZOA TO A HDG OF 260 DEGS AND DSCNT CLRNC TO 13000 FT. AFTER SEVERAL MINS ON THE 260 DEG HDG WE WERE GIVEN A RAPID SERIES OF CLRNCS: 'DIRECT EUGEN; HADLY 2 ARR; CROSS EUGEN ONE-ONE-THOUSAND AND 250 KTS.' THE FO AND I CONFIRMED BTWN OURSELVES THAT THE FIX WAS EUGEN AND THE ALT WAS 11000 FT AND I READ BACK 'DIRECT EUGEN; HADLY ARR 11000 FT.' HEARING NO OBJECTION FROM ATC I SET 11000 FT IN THE MODE CTL PANEL ALT WINDOW. THE PUBLISHED XING RESTR ON THE HADLY 2 STAR IS TO EXPECT EUGEN AT 11000 FT AND 250 KTS. AS WE APCHED EUGEN WE WERE BOTH INTENT ON ACCOMPLISHING THE DIFFICULT INTERCEPT AND XING RESTR. AT THIS TIME WE RECEIVED A TCASII ALERT OF TFC BELOW US. AS WE DSNDED OUT OF 12000 FT WE WERE TOLD BY ATC TO 'MAINTAIN PRESENT ALT.' THE FIRST INSTRUCTIONS WE HAD HEARD FROM ATC SINCE THE CLRNC DIRECT EUGEN. AFTER THAT WE WERE CLRED FOR THE HADLY 2 ARR; 12000 FT AND TURNED OVER TO BAY APCH. BAY APCH REQUESTED THAT WE CALL ZOA AFTER LNDG. WHEN WE CALLED WE WERE TOLD THAT ZOA HAD HAD A 'LOSS OF SEPARATION' AND THAT THEY SHOWED THAT WE HAD ONLY BEEN CLRED TO 13000 FT ON THE STAR AT THAT TIME. UP UNTIL THE TIME OF THE TCASII ALERT; BOTH THE FO AND I BELIEVED THAT WE WERE COMPLYING WITH ATC INSTRUCTIONS TO CROSS EUGEN AT 11000 FT. WE THOUGHT THAT WE HEARD AND READ BACK EUGEN AT 11000 FT. THE ATC METHOD OF USING 'ONE-ONE' AND 'ONE-THREE' PHRASEOLOGY RATHER THAN THE MORE CLEARLY DIFFERENTIATED 'ELEVEN' AND 'THIRTEEN' MAY HAVE BEEN A PROB WITH A FIX AT WHICH THE EXPECTED XING RESTR IS 11000 FT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR INDICATED THAT IN THE CALL TO THE CTR SUPVR; HE HEARD A PORTION OF THE TAPE WHERE THE CTLR ISSUED THE DSCNT CLRNC TO 13000 FT AND NOT 11000 FT AS THEY THOUGHT THEY HEARD.