NON ADHERENCE TO ATC CLRNC INSTRUCTION.

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SEA ATIS ANNOUNCED RWY 34L AS DEP RWY. GND TOLD US TO TAXI TO RWY 34L; HOLD SHORT OF RWY 34R AT INTXN Q. APCHING INTXN Q WE WERE CLRED TO TAXI ACROSS RWY 34R; CLRED FOR TKOF RWY 34L. WE ADVISED TWR WE WERE NOT READY BECAUSE WE DIDN'T HAVE OUR FINAL NUMBERS FOR WT AND BAL. WE WERE TOLD TO HOLD SHORT OF RWY 34R. LATER; WHEN WE ADVISED TWR WE WERE READY; WE HEARD 'TAXI INTO POS AND HOLD RWY 34L. TFC 6 MI OUT FOR THAT RWY. TFC ON A L BASE FOR RWY 34R.' THE FO READ BACK 'INTO POS AND HOLD RWY 34L.' WHILE XING RWY 34R AT TXWY Q; I SAW THE 6 MI FINAL FOR RWY 34R. WHILE APCHING RWY 34L; I SAW A SMALL AIRPLANE ON A L BASE. HOWEVER; IT APPEARED THAT HE WAS GOING TO RWY 34L. I SLOWED MY TAXI AND WAS GOING TO QUERY THE TWR PRIOR TO ENTERING RWY 34L; BUT TWR THEN CALLED AND SAID HOLD SHORT RWY 34L. TWR THEN ASKED 'WHAT DON'T YOU UNDERSTAND ABOUT POS AND HOLD RWY 34R?' WITHOUT REPLAYING THE TAPES; I'M NOW NOT SURE WHAT EXACTLY WAS SAID. THINKING BACK; I DIDN'T HEAR TWR SAY 'CROSS RWY 34R; TAXI INTO POS AND HOLD;' WHICH WAS A CLUE. BUT I ALSO DIDN'T HEAR 'TAXI INTO POS AND HOLD ON RWY 34R AT INTXN Q' EITHER; WHICH WOULD BE REQUIRED FOR AN INTXN TKOF. I; THE FO; AND A COCKPIT OBSERVER ALL THOUGHT THE TWR SAID POS AND HOLD RWY 34L; BUT IT IS POSSIBLE THAT THE TA'S CONCERNING THE L BASE TO RWY 34L LED US TOWARD THE L RWY. IN SUMMARY; 3 OF US HEARD 'INTO POS AND HOLD RWY 34L;' WHETHER THAT WAS IN FACT THE CASE OR NOT. THE FO READ BACK 'INTO POS AND HOLD RWY 34L' WHICH; IF WAS NOT OUR CLRNC; SHOULD HAVE BEEN CAUGHT BY THE TWR. IF WE WERE CLRED ONTO HOLD FOR RWY 34R; THE CLRNC SHOULD HAVE INCLUDED THE INTXN; SINCE THE APCH END OF RWY 34R IS 2500 FT FURTHER DOWN THE RWY. WE DIDN'T HEAR 'CROSS RWY 34R; TAXI INTO POS AND HOLD RWY 34L' WHICH; IF WE THOUGHT WE WERE TO BE ON RWY 34L; SHOULD HAVE PROMPTED US TO QUESTION THE CLRNC. BECAUSE WE CHKED FINAL APCHS WHEN XING AND APCHING THE RWYS; WE SAW A POTENTIAL CONFLICT AND WERE ABLE TO AVOID TAXIING ONTO A RWY THAT ANOTHER ACFT WAS ABOUT TO LAND ON.