ALTDEV ALT UNDERSHOT. FLC OF ACR LEVELED OFF DURING A NON DISCRETIONARY DSCNT.

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CTR CTLR GAVE CLRNC TO DSND SO AS TO CROSS 55 MI N OF MARVI INTXN AT FL270. NEXT CTLR STATED TO COMPLY WITH PREVIOUS RESTRS AND DSND AND MAINTAIN FL180. MY INTERP WAS THAT THE FL180 CLRNC WAS AT PLT'S DISCRETION AFTER THE FL270 RESTR. THAT WAS NOT WHAT THE CTLR SAID OR WANTED. THE FMC LEVELED THE ACFT AT FL270 AND SHOWED 15 NM BEFORE THE OPTIMUM TOP OF DSCNT POINT. SO EVEN THOUGH THE ALT WINDOW HAD FL180 SET IN; THE ACFT TEMPORARILY LEVELED OFF. THE CTLR ASKED WHAT THE RATE OF DSCNT WAS; AND WE TOLD HIM WE LEVELED OFF AT FL270. HE SAID DSND TO FL180 AND TURN L HDG XXX FOR RADAR VECTORS. HE DID NOT SAY IF THERE WAS A CONFLICT AND AFTER A SHORT VECTOR HE TURNED US BACK ON COURSE AND NOTHING FURTHER WAS MENTIONED. THIS RESULTED FROM AN ASSUMPTION THAT THE FL180 CLRNC WAS DISCRETIONARY. NEITHER MYSELF NOR THE FO UNDERSTOOD IT THAT WAY BECAUSE IT WAS NOT WHAT WE EXPECTED TO HEAR. THE CTLR WAS DOING HIS JOB WELL AND THE PLTS WERE NOT. EVEN THOUGH WE READ THE CLRNC BACK CORRECTLY; WE DID NOT COMPLY WITH WHAT WE READ BACK. WHEN THE ACFT LEVELED OFF I CONSIDERED SELECTING ANOTHER AUTOPLT MODE TO CONTINUE THE DSCNT; BUT DID NOT. THE FO DID NOT QUESTION MY ACTIONS EVEN THOUGH WE COVERED EXACTLY THIS SCENARIO IN OUR PREFLT BRIEFING ON THE FIRST DAY OF TRIP. FO WAS LACKING IN MONITORING AND ASSERTIVENESS AND THE CAPT (ME) LACKED SUPERVISION QUALITIES AND DID NOT ASK THE FO FOR CLARIFICATION WHEN SOMETHING JUST DIDN'T FEEL RIGHT WHEN I WAS CONSIDERING ANOTHER AUTOPLT MODE TO CONTINUE THE DSCNT. (COCKPIT RESOURCE MGMNT NOT EFFECTIVELY USED.)