BEING SURE OF THEIR ALT CLRNC AND SETTING THE ALERTER PER COMPANY PROTOCOL; A CREW QUESTIONS THE FACT THAT ATC IS NOW QUESTIONING THEIR ALT JUST BEFORE LEVELOFF AS BEING ABOVE THEIR CLRNC ALT.

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AT 16000 FT ENRTE TO MLO; ZMA ISSUED A CLB CLRNC TO FL210. WE CHANGED OUR ALT SELECTOR TO '210;' ADJUSTED OUR CABIN PRESSURE CTL TO FL210 AND INITIATED A CLB PROFILE TO FL210. AT FL206; ZMA STATED THAT WE WERE 600 FT HIGH AND ASKED OUR ASSIGNED ALT. WE REPLIED 'FL210.' ZMA ADVISED US OUR ASSIGNED ALT WAS FL200 AND TO DSND; WHICH WE DID. MY FO AND I WERE SURE OUR ORIGINAL ASSIGNED ALT WAS FL210. WE READ IT BACK. ONE PLT (THE FO) ADJUSTED THE ALT SELECTOR TO FL210 AND THE CAPT (ME) STATED 'FL210 CONFIRMED.' THAT FACT AND OUR FL210 PRESSURIZATION ADJUSTMENT MADE IT OBVIOUS THAT WE WERE 'SURE' OF OUR ASSIGNED ALT. MY PERSONAL POLICY IS THAT IF EITHER PLT IS UNSURE; WE RE-QUERY THE CTLR AS TO THE INSTRUCTION. WE WERE SURE. I KNOW THAT CTLRS ARE VERY BUSY AT TIMES. IT WOULD RESULT IN MASS CONFUSION IF EVERY FLT QUESTIONED EVERY CLRNC THAT DIDN'T MAKE SENSE TO THE PLT. SO WE ONLY QUESTION THE ONES WE'RE UNSURE OF THE MEANING OF. IF IT'S SAFE; POSSIBLE AND CLR; WE DO IT. THIS JUST REINFORCES THE NEED TO BE SURE OF WHAT WE HEAR; VERIFY CLRNCS WITH THE OTHER PLT (WHEN WE HAVE ONE); QUERY CTLRS WHEN CLRNCS ARE UNCLR AND FOR CTLRS TO LISTEN TO AND BE SURE THEY UNDERSTAND READBACKS FROM PLTS IN CASE ALL THE PREVIOUS EFFORTS STILL DON'T WORK. DISCIPLINE IN STANDARDIZED COM IS CRITICAL TO PROFESSIONALISM.