AN ACR FLT DSNDS BELOW THE STAR RELATED ALT PRIOR TO THE 15 DME SW OF YWT FOR A PROFILE DSCNT INTO CYYZ; ON.

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WE DEPARTED IND WITH A RESTR OF CYYZ. APCHING YXU; ZOB CLRED US TO CYYZ VIA YXU DIRECT YWT DIRECT YYZ. SHORTLY AFTER PASSING YXU; TORONTO CTR INSTRUCTED US TO DSND TO FL190 INITIALLY; THEN DOWN TO 12000 FT. WHILE IN THE DSCNT TO 12000 FT; WE WERE CLRED TO 'DSND TO 7000 FT VIA THE RWY 5&6R PROFILE DSCNT.' THE FO WAS SERVING AS PF. I REPEATED THE CLRNC TO HIM AND TOLD HIM I WOULD INSERT THE PROFILE DSCNT ROUTING INTO THE FMS. WE DID NOT REVIEW THE ALT AND AIRSPD RESTRS ON THE PROFILE DSCNT AT THAT TIME. I HAD MENTIONED THE PROBABILITY OF RECEIVING CLRNC FOR THE PROFILE DSCNT AND ASSUMED HE HAD REVIEWED IT INSTEAD. I MADE THE MISTAKE OF IMMEDIATELY GOING HEADS DOWN TO REPROGRAM THE FMS. THE PROFILE DSCNT CALLED FOR US TO CROSS 15 NM SW OF YWT BTWN 14000 FT AND 12000 FT AND THEN CONTINUE THE DSCNT TO CROSS 9 NM NE OF YWT (26 DME YYZ) AT 7000 FT AND 210 KIAS. I DO NOT RECALL CONFIRMING ANY CHANGE TO THE ALT SET ON THE FLT CTL PANEL WITH THE FO (NOR BEING ASKED TO CONFIRM ANY ALT CHANGE). HOWEVER; WHEN I LOOKED UP FROM PROGRAMMING THE FMS; WE WERE DSNDING AND APCHING 10000 FT WHILE STILL APPROX 25 NM SW OF YWT (10 NM PRIOR TO THE POINT WE COULD DSND BELOW 12000 FT!). I IMMEDIATELY INSTRUCTED THE FO TO LEVEL OFF AND INFORMED TORONTO CTR WE WERE BELOW THE PROFILE DSCNT. THE CTLR APPEARED UNCONCERNED AND RECLRED US TO MAINTAIN 10000 FT AND DSND TO 7000 FT ON THE PROFILE DSCNT. THE REMAINDER OF THE DSCNT; APCH; AND LNDG WERE UNEVENTFUL. WE HEARD NOTHING ELSE FROM THE CTLR REGARDING OUR APPARENT ALTDEV. AFTER LNDG; THE FO AND I DISCUSSED WHAT HAPPENED. HE UNDERSTOOD OUR CLRNC TO BE DSND TO 7000 FT AND THEN CLRED FOR THE PROFILE DSCNT. IF HE WAS CORRECT; NO ALTDEV OCCURRED. IF MY UNDERSTANDING WAS CORRECT; WE DSNDED TOO EARLY. REGARDLESS; WE BOTH LEARNED SEVERAL IMPORTANT LESSONS (AGAIN!). FIRST; IT IS IMPERATIVE THAT WE BOTH UNDERSTAND AND AGREE ON OUR CLRNC. HAD WE DISCUSSED IT AT THE TIME; WE COULD HAVE CLARIFIED OUR DIFFERENT UNDERSTANDINGS WITH ATC AND AVOIDED ANY POTENTIAL PROB ALTOGETHER. SECOND; AS PNF; IT IS MUCH MORE IMPORTANT TO REVIEW OUR CLRNC AND BACK UP THE PF THAN TO BE IMMEDIATELY CONCERNED WITH REPROGRAMMING THE AUTOMATION. BETTER TO RESORT TO RAW DATA THAN BUST AN ALT. FINALLY; IF WE HAD FOLLOWED FLT MANUAL POLICY OF CONFIRMING CHANGES TO ALT SET ON THE FLT CTL PANEL; WE COULD HAVE DISCOVERED OUR MISUNDERSTANDING AND PREVENTED A POSSIBLE DEV. THEY POLICY IS THERE FOR A REASON!