ZDC CTLR APPLIES VISUAL SEPARATION BTWN A DEPARTING RDU CL65 AND ANOTHER ENRTE JET WHICH IS LATER QUESTIONED BY THE CL65 FLC.

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WE WERE LEVEL AT 17000 FT MSL TALKING TO CTR WHEN HE POINTED OUT TFC OFF TO OUR R. (WE WERE ON THE SID OUT OF RDU PROCEEDING TO LIB.) THE TFC WAS ABOVE US AND APPROX 2.5 MI AWAY. WE WERE TOLD TO MAINTAIN VISUAL SEPARATION WITH THE TFC AND HE WAS GIVEN A DSCNT THROUGH OUR ALT. AT THE TIME WE THOUGHT HE WAS PARALLELING OUR COURSE. AFTER A MIN; IT BECAME EVIDENT THAT HE WAS NOT PARALLELING; BUT CONVERGING WITH US. WE ASKED THE CTLR WHAT HE WAS DOING AND HE SAID HE WAS JOINING THE SID IN FRONT OF US. WE HAD NOT BEEN INFORMED OF THIS AND THERE WAS NO WAY WE COULD MAINTAIN SEPARATION ON OUR CURRENT HDG. AT THAT TIME WE ASKED FOR A TURN; AND A FEW SECONDS LATER WE GOT A TA AND ALSO ADVISED ATC OF THIS. HIS RESPONSE WAS; 'ARE YOU REQUESTING A TURN?' I DON'T KNOW HOW HE COULD NOT SEE THAT WE WERE ON A CONFLICTING COURSE AND HOW HE THOUGHT WE COULD STILL MAINTAIN VISUAL AND SEPARATION. AT THIS POINT; THE OTHER ACFT VERIFIED THAT THEY HAD BEEN GIVEN A DSCNT; AND THE CTLR'S RESPONSE WAS; 'YES. THE OTHER ACFT WAS SUPPOSED TO MAINTAIN A VISUAL ON YOU.' WE DID HAVE A VISUAL ON HIM; BUT THE CTLR HAD GIVEN US NO CHOICE; BUT TO CHANGE HDG BECAUSE MAINTAINING THE SID WOULD HAVE PUT US TOO CLOSE TOGETHER. I STILL DO NOT KNOW WHAT THE CTLR WAS THINKING WHEN HE GAVE US THOSE INSTRUCTIONS. I BELIEVE THE OTHER ACFT WAS AN ACR X JET; AND THE SEPARATION AT THE TIME OF OUR HDG CHANGE WAS GIVING US A TA; BUT WE NEVER HAD AN RA.