ACR FLC MISINTERPRETED A DSCNT CLRNC AND LEVELED OFF AT AN INTERMEDIATE ALT. NON ADHERENCE TO ATC CLRNC INSTRUCTION.

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OUR CLRNC WAS 'CROSS HYP AT FL240 OR BELOW; PAPEE AT 16000 FT OR BELOW; DSND AND MAINTAIN 12000 FT. THE CAPT READ BACK THE CLRNC AS GIVEN BY ZOA. THE ACFT WAS IN VNAV AND PROGRAMMED TO MAKE THE XING RESTRS WITH THE AUTOPLT ON. BOTH THE RESTRS WERE MADE AND THE ACFT LEVELED AT 16000 FT AT PAPEE. WE WERE LEVEL FOR ABOUT A MIN WHEN ZOA ASKED IF WE HAD LEVELED AT 16000 FT. BECAUSE THE CAPT WAS MAKING A PA; I RESPONDED THAT WE WERE LEVEL. ZOA REITERATED THAT THE CLRNC WAS TO DSND TO 12000 FT AND TO EXPEDITE THAT DSCNT. I EXPEDITED THE DSCNT TO 12000 FT AND LEVELED THERE. I HAD MISINTERPRETED THE INITIAL CLRNC TO DSND ALL THE WAY TO 12000 FT. MY INITIAL INTERP WAS PLT'S DISCRETION TO 12000 FT AFTER PAPEE. AS I LEVELED AT 16000 FT THE CAPT ASKED IF I UNDERSTOOD THE DSCNT CLRNC. I RESPONDED THAT I DID; AND TO LEVEL OFF. MORE COM WAS NEEDED IN THE COCKPIT. I ALSO SHOULD HAVE QUESTIONED THE CLRNC AS SOON AS THE CAPT ASKED IF I UNDERSTOOD THE CLRNC. RELIANCE ON THE FMC TO MEET ALL RESTRS WAS ALSO A MAJOR FACTOR. SUPPLEMENTAL INFO FROM ACN 316850: ACFT CLRED INTO SJC VIA EL NIDO 3 ARR. AT PAPEE (JUST PRIOR) I NOTICED ACFT LEVELING AT 16000 FT. I ASKED FO WHO HAD DONE PROGRAMMING AND WAS PF WHY ACFT WAS LEVELING. HE AFFIRMED THAT XING WAS AT OR BELOW. I CONCURRED AND NOTED 12000 FT WAS THE ALT CLRED TO. I THEN PROCEEDED TO DO THE PA TO THE PAX. DURING THE PA I SAW WE WERE STILL AT 16000 FT AND POINTED TO THE ALT ALERT SET AT 12000 FT. FO FOR SOME REASON HAD THOUGHT THE CLRNC TO 12000 FT AFTER THE XING AT 16000 FT WAS AT PLT'S DISCRETION. WE DISCUSSED THE FACT THAT PLT'S DISCRETION WAS NEVER MENTIONED BY THE CTLR OR BY MYSELF DURING THE READBACK OR BY THE FO IN HIS READBACK. IN THE FUTURE I AM GOING TO HAVE TO BE MORE SPECIFIC AND MORE ON TOP OF THE FO'S FLYING. AFTER FLYING WITH SOMEONE FOR A NUMBER OF TRIPS IT'S EASY TO GET COMPLACENT AS TO THEIR FLYING PROCS.