A PA60 PLT; BEFORE DEPARTING HEF ON AN IFR CLRNC; RECEIVED AN AMENDMENT THAT WAS UNCLEAR; AND WAS NOT WHAT PCT EXPECTED.

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MANASSAS GND CLRED ME TO MAINTAIN RWY HDG TO 800 FT (USING RWY 16L); THEN DIRECT CSN; CROSS 10 MI W OF CSN AT 2000 FT; THEN CSN 3000 FT. MANASSAS TWR CLRED ME FOR TKOF. WHEN I WAS ON THE RWY; BEFORE BRAKE RELEASE; TWR ADVISED THAT DEP WANTED TO CHANGE MY CLRNC TO MAINTAIN RWY HDG TO 2000 FT. I READ BACK THESE INSTRUCTIONS. I ASSUMED THIS WAS MEANT TO MAINTAIN THE REST OF MY DEP CLRNC. SO; I MAINTAINED RWY HDG UNTIL 2000 FT; THEN TURNED TOWARDS CSN. I WAS XFERRED TO POTOMAC APCH/DEP RIGHT BEFORE REACHING 2000 FT; AND THEY ANSWERED MY CALL SOON AFTER I HAD LEVELED OFF AT 2000 FT AND HAD JUST ABOUT COMPLETED MY R TURN TOWARDS CSN. THE CTLR WAS SURPRISED THAT I WAS TURNING AND HAD EXPECTED ME TO MAINTAIN RWY HDG; EVEN AFTER 2000 FT. IT SOUNDED LIKE HE DID NOT WANT ME HDG TOWARDS CSN AT ALL. I THINK THAT CLRER INSTRUCTIONS FROM HEF TWR THAT LED ME TO A PREVIOUS SEGMENT OF THE CLRNC; OR THAT TOLD ME TO CLB TO 2000 FT AND MAINTAIN RWY HDG WOULD HAVE CLRED THINGS UP. EVEN BETTER WOULD HAVE BEEN FOR THE TWR TO INSTRUCT ME TO DISREGARD THE CLRNC PORTION THAT SENT ME TO CSN; OR TO EXPECT VECTORS AT SOME POINT. POTENTIALLY ADDING TO THE CONFUSION WAS THE FACT THAT I WAS ALREADY ON THE RWY; READY TO START ROLLING FOR TKOF; WHEN I GOT THE CHANGE IN THE CLRNC. I DIDN'T THINK THIS SHOULD HAVE BEEN A BIG FACTOR HAD THE TWR'S INSTRUCTIONS BEEN CLRER.