BTV CL65 DEP; EXPERIENCED CONFUSION WITH FULL LENGTH RWY TKOF.

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GND/TWR (SAME CTLR) INSTRUCTED US TO TAXI TO RWY 33 BEHIND A DORNIER. AS WE PROCEEDED ALONG TXWY C; THE CTLR TOLD US TO HOLD SHORT RWY 33 AT TXWY C (WHILE THE DORNIER CONTINUED ALONG TXWY K TO HOLD SHORT RWY 33 AT TXWY J). WE ACKNOWLEDGED OUR CLRNC WITH HESITATION BECAUSE OUR RWY PERFORMANCE NUMBERS ARE BASED ON USING THE ENTIRE 8320 FT TKOF LENGTH; INCLUDING THE DISPLACED THRESHOLD. TXWY C INTERSECTS RWY 33 APPROX 200 FT NW OF THE BEGINNING OF THE DISPLACED THRESHOLD. ATC TOLD US THAT WE COULD ACCESS FULL LENGTH FROM TXWY C. BECAUSE IT WAS OBVIOUS THAT FULL LENGTH DID NOT START AT TXWY C; I THOUGHT ATC'S COMMENT MEANT THAT THE CTLR UNDERSTOOD WE WOULD NEED TO BACK TAXI INTO POS FULL LENGTH. ATC CLRED US FOR TKOF; BUT WE WERE NOT READY AND CONTINUED TO HOLD SHORT WHILE THE DORNIER DEPARTED. WHEN READY; THE FO TOLD ATC THAT WERE READY FOR TKOF; FULL LENGTH; RWY 33. ATC TOLD US TO TAXI INTO POS AND HOLD. WE ENTERED RWY 33 AT TXWY C AND BACK-TAXIED TO THE BEGINNING OF THE DISPLACED THRESHOLD. AS WE TURNED BACK TOWARD THE NW; ATC HAD TOLD US TO FLY THE RWY HEADING; CLRED FOR TKOF. THERE WAS NO OTHER KNOWN INBOUND OR OUTBOUND TFC. I PROBABLY SHOULD HAVE HELD AT TXWY C UNTIL I ACTUALLY HEARD THE CTLR USE THE TERMINOLOGY 'BACK TAXI' INTO POS AND HOLD RWY 33. IN THE FUTURE; I WILL. USUALLY THE DISPLACED THRESHOLD IS NOT AN ISSUE BECAUSE I TAXI VIA TXWYS C; K; J.