A B737 FLC DSNDS TO 7000 FT. ATC ASKS IF THEY ARE GOING TO 6000 FT. WITH NOTHING FURTHER SAID; THE PIC DSNDS TOWARDS 6000 FT AND IS TOLD TO MAINTAIN 7000 FT.

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WE WERE ON APCH TO BWI IN DAYLIGHT; VFR; TALKING TO APCH CTL. WE WERE ISSUED A CLRNC TO 7000 FT AND WERE LEVEL AT 7000 FT FOR SEVERAL MINS. THE CTLR ASKED US TO 'CONFIRM WE WERE GOING DOWN TO 6000 FT.' WE SAID 'NO;' AND ALSO TOLD HIM 'WE WERE ISSUED 7000 FT.' HE PROCEEDED TO TALK TO OTHER ACFT ON THE FREQ AND LEFT US VERY CONFUSED. THINKING WE WERE SUPPOSED TO BE AT 6000 FT; THE CAPT STARTED DSNDING. I GOT ON THE RADIO AND ASKED TO CONFIRM OUR ASSIGNED ALT. HE SAID '7000 FT.' THE CAPT HAD ALREADY DSNDED TO 6600 FT AND MADE AN IMMEDIATE CORRECTION TO 7000 FT. NOTHING ELSE WAS MENTIONED ABOUT IT AND NO EVASIVE ACTION WAS REQUIRED. THE CTLR LEFT US VERY CONFUSED AND UNCOMFORTABLE AND WE WEREN'T COMPLETELY SURE WHAT HE WANTED FROM US. SUPPLEMENTAL INFO FROM ACN 415675: WE HAD PASSED NOTTINGHAM AND WERE PROCEEDING TOWARDS DEALE. NORMAL CLRNC IS TO HAVE BEEN CLRED TO 6000 FT AT DEALE SO THIS FELT A LITTLE ABNORMAL. THE RADIO WAS VERY BUSY. SEVERAL OTHER RADIO XMISSIONS OCCURRED BEFORE WE COULD FINALLY QUERY. AT THIS POINT WE HAD DSNDED TO 6650-6700 FT AND IMMEDIATELY RETURNED TO 7000 FT. AT NO TIME WAS SAFETY COMPROMISED BUT THERE WAS A POTENTIAL TFC CONFLICT AT 6000 FT HAD WE CONTINUED. 2 FACTORS LED TO THIS POTENTIAL CONFLICT: 1) THE ASSUMING NATURE OF DCA CONCERNING OUR ALT AND WHEN IT DISAGREED WITH OUR RESPONSE NO FOLLOW-UP CLARIFICATION OF WHAT WAS THE CORRECT CLRNC. 2) OUR ASSUMPTION DCA WAS CORRECT AND WE NEEDED TO CHANGE ALTS. LESSONS LEARNED: FOR ME; NEVER ASSUME. WHEN A CONFLICT EXISTS I SHOULD MAINTAIN WHAT I BELIEVE IS MY LAST CLRNC UNTIL CLARIFICATION/CONFIRMATION CAN BE OBTAINED. FOR ATC; DON'T LEAVE A QUESTION/CONFUSION ON A CLRNC OPEN. ONCE WE HAD GIVEN A RESPONSE THAT CONFLICTED WITH HIS ASSUMPTION HE SHOULD HAVE IMMEDIATELY CLARIFIED OUR CORRECT CLRNC BEFORE GOING ON TO OTHER TFC.