AN RJ85 CREW BACK TAXIED ON THE ACTIVE RWY AFTER MISTAKING A TAXI CLRNC.

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WE WERE GIVEN RWY 6L FOR DEP IN CLE. ONCE ON TXWY G; WE WERE TOLD TO HOLD SHORT OF RWY 6L AT TXWY T FOR DEP. (THERE WAS AN ACFT HOLDING AT THE END NOT READY FOR DEP.) THERE WAS 1 ACFT AHEAD OF US AT TXWY T THAT WAS TOLD TO TAXI INTO POS FOR INTXN DEP. THEY ADVISED THAT THEY DIDN'T HAVE NUMBERS FOR AN INTXN DEP AND NEEDED FULL LENGTH. THE LCL CTLR TOLD THEM TO BACK-TAXI FULL LENGTH FOR DEP; AND THEY WERE CLRED FOR TKOF. WE WERE THEN ISSUED CLRNC TO TAXI INTO POS AND HOLD FOR INTXN T DEP. WE ADVISED THAT WE NEEDED FULL LENGTH AS WELL AND THE TWR CTLR TOLD US TO TAXI TO THE END. I HAD ALREADY BEGUN TO MOVE THE ACFT ONTO THE RWY WHEN WE RECEIVED THE CLRNC TO TAXI TO THE END. I UNDERSTOOD THAT TO MEAN BACK-TAXI INTO POS. I HAD VISUALLY CLRED THE FINAL APCH AND DID NOT SEE ANY ACFT ON FINAL. DURING THE BACK-TAXI; I NOTICED AN ACFT ON FINAL BUT WASN'T SURE WHICH RWY THEY WERE ALIGNED FOR UNTIL THE CTLR ISSUED A GAR TO THAT ACFT. THE CTLR THEN ADVISED US THAT HE WANTED US TO TAXI TO THE END VIA TXWY G FOR A FULL LENGTH DEP. FROM MY STANDPOINT; HAVING SEEN THE ACFT JUST PRIOR TO US BE GIVEN FULL LENGTH BACK-TAXI; I THINK I WAS ALSO ANTICIPATING A BACK-TAXI AND WHEN THE CLRNC WE RECEIVED WASN'T CONCISE; I ASSUMED HE MEANT FOR US TO BACK-TAXI. (INCORRECT ASSUMPTION.) ALSO; IF I HAD SEEN THE ACFT ON FINAL; I MAY HAVE QUESTIONED THE INSTRUCTIONS; HOWEVER; THE RWYS IN CLE ARE VERY CLOSE TOGETHER AND IT CAN BE DIFFICULT TO KNOW WHICH RWY ACFT ON FINAL ARE USING. THIS WAS A VERY BUSY TIME FOR THE CTLR AND WHEN HE ISSUED US THE CLRNC TO 'TAXI TO THE END'; HE ASSUMED THAT WE WOULD USE THE TXWY G TO THE END OF THE RWY. HIS CLRNC SHOULD HAVE BEEN EITHER HOLD SHORT OR TAXI TO THE END VIA TXWY G. TO HIS CREDIT; HE NOTICED WE WERE ON THE ACTIVE RWY AND ISSUED GAR TO THE ACFT ON A 1-2 MI FINAL.