B737 TCASII RA WITH PROBABLE VFR CESSNA. EVASIVE ACTION TAKEN; SEE AND AVOID CONCEPT.

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I WAS THE PLT FLYING WHILE CONDUCTING THE JASEN 1 ARR BECKLEY TRANSITION TO IAD. I WAS LEVELING AT 11000 FT AND 250 KTS 3 TO 4 MILES W OF DOCCS ON THE AML 259 DEG RADIAL WHEN ATC CLRED US 'TURN TO 060 DEG AND INTERCEPT THE JASEN 1; DSND AND MAINTAIN 5000 FT.' HAVING RECEIVED THIS CLRNC W OF THE LDN 206 DEG RADIAL; IT WAS THE NEXT PART OF THE ARR THAT WOULD BE INTERCEPTED AFTER TURNING TO 060 DEG. WHILE DSNDING THROUGH APPROX 9200 FT AND TURNING TO INTERCEPT THE LDN 206 DEG RADIAL; WE RECEIVED A TCASII RA TO CLB. I PERFORMED THE MANEUVER WHICH TOOK US BACK UP TO 10000 FT. WE IMMEDIATELY ADVISED ATC AND HE CLRED US TO TURN TO 080 DEG AND JOIN THE ARR ON THE 300 DEG RADIAL W OF AML. THE REMAINDER OF THE FLT WAS UNEVENTFUL. IN LIGHT OF WHAT HAPPENED; I NOW BELIEVE THAT THE CTLR WANTED US TO INTERCEPT THE ARR W OF AML ON THE 300 DEG RADIAL FROM THE BEGINNING. HOWEVER; SINCE WE WERE GIVEN THE CLRNC W OF THE LDN 206 DEG RADIAL; IT WAS THE FIRST POINT OF INTERCEPT AFTER TURNING TO 060 DEGS. HAD THE CTLR PRECEDED THE CLRNC WITH 'AFTER DOCCS' OR BEEN SPECIFIC ABOUT WHERE HE EXPECTED US TO RE-JOIN; THERE WOULD HAVE BEEN NO QUESTION AS TO WHAT WAS EXPECTED.