A DSNDING B737-300 FLC QUESTIONS THE TECHNIQUE AND CLRNC AS EVINCED BY THE ZDC CTLR DURING A POTENTIAL CONFLICT WITH DEPARTING; CLBING TFC 20 MI E OF CSN; VA.

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I (FO) WAS FLYING THE ACFT AND CAPT WAS HANDLING THE RADIOS ON FLT FROM BNA TO BWI. WE WERE APPROX 20 NM E OF CSN VOR ON THE OTT 5 ARR AT FL190 AND WERE GIVEN DSCNT CLRNC TO 12000 FT BY ATC WHICH WAS READ BACK BY CAPT. MY CONCERN IS BECAUSE OF THE TONE OF VOICE FROM ZDC WHEN SHE ASKED 'WHAT ALT WERE WE DSNDING TO?' WE RECEIVED A TA ALERT FROM A JET XING FROM N OF OUR POS; BUT NEVER WAS THE TA CLOSER THAN 5 NM. DURING THE DSCNT; PASSING APPROX 13500 FT; ATC ASKED 'WHAT ALT ARE YOU DSNDING TO?' WHICH THE CAPT REPLIED '12000 FT.' VERY SHORTLY THEREAFTER (ABOUT 10 SECONDS) ATC GAVE US A 20 DEG R TURN FOLLOWED BY INSTRUCTIONS TO DSND TO 9000 FT -- ALL OF WHICH WERE READ BACK BY CAPT. AFTER APPROX 3 MINS; WE WERE TOLD BY ATC TO DSND TO 6000 FT AND PROCEED DIRECT DEALE INTXN. WE COMPLETED A VISUAL APCH PROC TO RWY 33L AT BWI. SUPPLEMENTAL INFO FROM ACN 493197: VECTORED OFF STAR IN VICINITY OF CSN VOR. LATER GIVEN CLRNC DIRECT TO OTT. GIVEN A DSCNT TO 12000 FT; WHICH WAS SET IN ALT ALERT WINDOW. WE RECEIVED A TA ON AN ACFT N OF US BY 5-6 MI WHICH WAS CLBING OUT. ARTCC GAVE THIS ACFT A TURN AND AT THIS TIME I NOTICED THE ACFT CLBING OUT OF THE UNDERCAST WHICH HAD TOPS AT ABOUT 11000 FT MSL. WE WERE GIVEN A TURN AND NOTHING MORE WAS SAID ABOUT IT. MY FO; OUR NEW HIRE JUMP SEAT RIDER; AND I ALL AGREED THAT WE HAD INDEED BEEN CLRED TO 12000 FT.