DC10 FLC TAXIED INTO POS AND HELD WHEN THEY WERE ONLY CLRED TO HOLD SHORT AND BE READY.

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CREW UNDERSTOOD WE WERE CLRED FOR 'POSITION AND HOLD.' WE TOOK THE RWY BEHIND LANDING TFC WAITING TKOF CLRNC. TOWER INSTRUCTED US TO TAXI CLEAR; WE THOUGHT FOR LOSS OF ROOM AS LNDG ACFT ROLLED LONG. WE TAXIED BACK AND TOOK OFF. 4 HOURS LATER AFTER LNDG AT SEA WE WERE ADVISED THAT WE WERE NOT CLRED TO GO INTO POS AND HOLD. FACTORS: CREW WAS LATE RECEIVING WEIGHT DATA AND WAS PERFORMING COMPUTATIONS AND CHKLISTS APCHING RWY. CTLR (TOWER) WAS WORKING 2 RWY'S AND TRANSMITTING ON BOTH FREQ'S; CREW WAS HEARING CALLS TO BOTH RWYS. WE WERE TOLD VIA TRANSCRIPT TO 'TAXI UP; HOLD SHORT; BE READY;' WITH OTHER ITEMS AND CHKLISTS GOING ON THE CREW MISTOOK THIS TRANSCRIPTION TO 'POS AND HOLD AND BE READY.' APCHING OUR ROLLOUT AT HOLDSHORT LINE FROM PARALLEL TXWY WE HEARD TOWER CLR ANTHER PLANE TO LAND; AND THAT A 'DC10 WILL BE IN POS.' THE 'BE READY ' AND 'DC10 IN POS'...TASK SATURATION AT THE TIME; TRANSMISSIONS/CHKLIST INTERRUPTED CONTRIBUTED TO THIS ERROR. DC10 CREWS ARE INTERNATIONAL. 'LINE UP AND WAIT; ETC.'.