FLC WAS ON ARR STAR; THE FMS STARTED TO LEAD THE TURN; ATC TOLD THEM TO FLY PRESENT HDG; WHICH WAS DIFFERENT FROM WHAT ATC THOUGHT IT SHOULD BE. ATC NEVER ASKED. ATC BELIEVES THIS RESULTED IN A PLTDEV.

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WE WERE ON THE SWEED 4 ARR INTO CINCINNATI; IN THE VICINITY OF SWEED. WE HAD BEEN CLRED TO CROSS DRESR INTXN AT FL240; WHICH WE DID. WE WERE THEN CLRED TO 11000 FT AND BEGAN OUR DSCNT. AS WE APCHED SWEED THE FMS COMMANDED THE ACFT TO BEGIN A TURN TOWARD FLM VOR. AT SOME POINT WE WERE ISSUED A CLRNC TO FLY OUR 'PRESENT HDG.' WE ENGAGED THE AUTOPLT HDG MODE. I DON'T RECALL THE EXACT HDG WE WERE ON AT THE TIME; BUT THE FO BELIEVES IT WAS 350 DEGS. LATER; WE WERE ASSIGNED A 010 DEG HDG FOR SEQUENCING. JUST AS WE APCHED FL190 WE WERE TOLD TO MAINTAIN FL190. WE ENGAGED THE ALT HOLD MODE OF THE AUTOPLT WHICH RESULTED IN A MOMENTARY DSCNT BELOW FL190 BEFORE LEVELING OFF AT FL190. THE CTLR THEN QUERIED OUR HDG. WE RESPONDED '010 DEGS ASSIGNED.' AT NO TIME WAS ANY TFC CALLED ALTHOUGH WE DID SEE TFC PASS OFF OUR L; VISUALLY AND ON TCASII 91200 FT BELOW US). WE NEVER RECEIVED ANY NOTIFICATION OF TFC FROM ATC. A FEW MINS LATER; WE WERE ASKED TO TELEPHONE ZID REGARDING A POSSIBLE PLT VIOLATION. WE CALLED AND WERE TOLD THEY BELIEVED US TO BE NBOUND ON THE SWEED ARR; S OF SWEED. WHEN WE WERE GIVEN THE CLRNC TO FLY OUR PRESENT HDG; WHICH SHOULD HAVE RESULTED IN A NBOUND HDG; INSTEAD WE HAD STARTED TO MAKE THE TURN AT SWEED WHICH RESULTED IN A MORE NORTHWESTERLY HDG. THEY SAID A LOSS OF IFR SEPARATION OCCURRED OF 4 1/2 MI AND 700 FT. SUGGESTIONS: 1) IF VECTORS ARE ASSIGNED FOR TFC; ATC SHOULD CLRLY STATE THIS AND EXPLAIN WHERE THE TFC IS. 2) INSTEAD OF GIVING A CLRNC TO MAINTAIN 'PRESENT HDG;' ASSIGN A SPECIFIC HDG IF THE ACFT IS CLOSE TO A TURN; OR ASK WHAT THE PRESENT HDG IS.