AN A319 CAPT; TAXIING FOR TKOF AT PIT; TAXIED INTO POS; WHILE FO WAS UNSURE IF THEY WERE CLRED TO DO SO.

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SO AS NOT TO DELAY ACFT TAXIING BEHIND US; THE CAPT HAD THE FO TELL PIT GND CTL THAT WE WOULD BE DELAYED A FEW MINS WHILE THE FLT ATTENDANTS COMPLETED THEIR SAFETY ANNOUNCEMENT. WE WERE INSTRUCTED TO HOLD SHORT OF TXWY W ON TXWY D; AND WE COMPLIED. A COUPLE OF MINS LATER; WE CALLED READY FOR TAXI AND PIT GND CTL ISSUED A TAXI CLRNC TO RWY 10C (AND PERHAPS A TAXI INTO POS CLRNC. THIS IS WHERE THE CONFUSION BEGINS.) THE CAPT CALLED FOR THE FINAL ITEMS OF THE BEFORE TKOF CHKLIST; WHICH THE FO PERFORMED HEADS DOWN; AS IS NECESSARY. UNDER THE IMPRESSION THAT WE HAD BEEN CLRED INTO POS; THE CAPT TAXIED ONTO THE OVERRUN SECTION OF RWY 28C WHICH IS SEVERAL HUNDRED FT FROM THE RWY 10C THRESHOLD. THE FO COMPLETED THE CHKLIST; LOOKED UP AND EXPRESSED CONCERN THAT WE HAD NOT BEEN CLRED INTO POS AND HOLD; WHEREAS THE CAPT WAS CERTAIN WE HAD. NEITHER THE CAPT NOR FO COULD RECALL THE EXACT WORDS FROM THE PIT GND CTLR; SO THE FO ANNOUNCED 'ACR X READY FOR TKOF'. THE TWR ISSUED A 'CLRED FOR TKOF' WITHOUT ANY APPARENT CONCERN; CONFUSION OR IRRITATION. WE; THEREFORE; CONCLUDED THAT WE HAD; IN FACT; BEEN ISSUED 'POS AND HOLD' CLRNC. THE TKOF CLRNC WAS ISSUED PRIOR TO US REACHING THE RWY 10C THRESHOLD. THE TKOF AND SUBSEQUENT HDOF TO DEP CTL CONTINUED NORMALLY WITHOUT COMMENT. SUPPLEMENTAL INFO FROM ACN 546734: CAPT CALLED FOR THE FINAL ITEMS OF THE BEFORE TKOF CHKLIST WHICH THE FO PERFORMED HEADS DOWN. THE CAPT TAXIED ONTO THE OVERRUN OF RWY 28C UNDER THE ASSUMPTION THAT WE WERE CLRED INTO POS RWY 10C AND HOLD. CAPTS SHOULD NOT CALL 'BELOW THE LINE' UNTIL TKOF OR POS AND HOLD SO THAT THE FO IS HEADS UP WHILE TAXIING ONTO THE RWY.