A POSSIBLE UNCLR XMISSION TO A HELI PLT BY SDF LCL CTLR WAS CONSTRUED AS AN INSTRUCTION.

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ON MAR/MON/01; I WAS IN CRUISE FLT SE OF SDF. THE ATC CTLR INSTRUCTED ME TO REMAIN E OF THE RWY 35L CTRLINE AT SDF. THE CTLR ANNOUNCED INCOMING TFC TO SDF FOR RWY 35L AND ASKED IF I HAD THE TFC IN SIGHT. I STATED I DID NOT HAVE TFC AND THAT I WAS SEARCHING. THE CTLR INSTRUCTED ME TO REMAIN E OF THE RWY 35L ARR PATH UNTIL I HAD THE INBOUND TFC IN SIGHT. I ALSO THOUGHT HER INSTRUCTIONS WERE THAT ONCE I HAD THE TFC IN SIGHT TO PASS BEHIND THE TFC AND CONTINUE ACROSS THE RWY 35L EXTENDED CTRLINE AS I CONTINUED SWBOUND OUT OF CLASS C AIRSPACE. I ESTABLISHED CONTACT WITH THE ARRIVING B737. I TURNED R TO PASS BEHIND THE B737 AND BEGAN TO CROSS THE RWY 35L EXTENDED CTRLINE AT SDF. ATC ANNOUNCED FOR ME TO TURN IMMEDIATELY TO THE E; TO GET BACK ON THE E SIDE OF THE RWY 35L EXTENDED CTRLINE. I THEN STATED TO ATC THAT I HAD VISUAL CONTACT WITH THE B737; AND I THOUGHT I WAS INSTRUCTED TO PASS BEHIND THE TFC AS I CONTINUED SWBOUND OUT OF THE CLASS C AIRSPACE. ATC SAID MY INSTRUCTIONS WERE TO REMAIN E OF THE RWY 35L EXTENDED CTRLINE AND THAT I WAS NOT GIVEN INSTRUCTIONS TO PASS BEHIND THE B737 OR CROSS THE EXTENDED CTRLINE OF RWY 35L. I APOLOGIZED TO ATC AND STATED THAT I MUST HAVE MISUNDERSTOOD HER INSTRUCTIONS. SHE STATED THAT WAS OK AND I CLRED THE CLASS C AIRSPACE TO THE SW. THE DISTANCE FROM THE B737 AS IT TURNED FROM BASE TO FINAL WAS ABOUT 1 1/2 MI FROM MY POS. CONTRIBUTING FACTORS: 1) MISCOMS. 2) COMPLACENCY/REPETITION; THE INSTRUCTION TO MAINTAIN SEPARATION AND PASS BEHIND TFC IS A RADIO CALL I HEAR ON A DAILY BASIS. ATC MAY NOT HAVE GIVEN ME THESE INSTRUCTIONS BUT AFTER HEARING IT OVER AND OVER; I MAY HAVE THOUGHT I HEARD THESE INSTRUCTIONS. CORRECTIVE ACTIONS: 1) ATC MADE THE CORRECTIVE ACTION. I THOUGHT I WAS IN THE RIGHT; ATC THOUGHT THERE WAS A CHANCE OF A MIDAIR COLLISION AND INSTRUCTED ME TO TURN AWAY FROM THE INCOMING TFC.