B747-400 ENTERS THE RWY AT WSSS WITHOUT CLRNC DUE TO CONFUSION REGARDING TERMINOLOGY.

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ON TAXI OUT FROM WSSS ENRTE VHHH NEARING THE DEP END OF RWY 20C GND ASKED 'WOULD WE BE READY WHEN WE REACHED THE RWY.' THE FO RESPONDED 'YES' AND WE WERE TOLD TO CONTACT TWR. INITIAL CONTACT TO THE TWR WAS; 'ACR X READY IN SEQUENCE.' THE NEXT XMISSION FROM THE TWR WAS 'POS RWY 20C.' THE FO RESPONDED 'POS 20 CTR.' AS WE TAXIED ONTO THE RWY THE TWR CLRED US FOR AN 'IMMEDIATE' TKOF. NOT LONG AFTER BEING AIRBORNE WE WERE ASKED WHY WE HAD ENTERED THE RWY (OR WORDS TO THAT EFFECT). AGAIN AT ALT; WE WERE SWITCHED TO WHAT MUST HAVE BEEN A FREQ NOT NORMALLY USED BY WSSS CTL AND ASKED ESSENTIALLY THE SAME QUESTION. WE REPLIED THAT WE HAD BEEN CLRED INTO 'POS' AND I ASKED FOR A TELEPHONE NUMBER TO CONTACT THEM UPON REACHING VHHH. AFTER A COUPLE OF CALLS TO WSSS CTL; I WAS EVENTUALLY TOLD THAT UPON INITIAL CONTACT THE TWR HAD SAID 'GOOD MORNING; ACR X HOLD SHORT OF RWY 20 CTR.' WE HAD RESPONDED 'POS RWY 20 CTR' AND THE TWR HAD SAID NOTHING IN RESPONSE. REGARDING HUMAN FACTORS; TO ME; THE INSTRUCTION 'POS' LIKE THE INSTRUCTION 'LINE UP AND WAIT' ARE UNIQUE TO INTL FLYING AND DIFFERENT ENOUGH FROM THE INSTRUCTION 'POS AND HOLD' (THAT BOTH THE FO AND I ARE USED TO) THAT I IMMEDIATELY HEAR THE DIFFERENCE AND PAUSE TO MAKE SURE I KNOW WHAT IS WANTED. IT SEEMS UNLIKELY THAT BOTH I AND THE FO WOULD HAVE CONFUSED THE WORDS 'HOLD SHORT' WITH THE NON STANDARD (TO US) PHRASE 'POS.' EQUALLY UNLIKELY WOULD BE THE FO'S RESPONSE TO THE WORDS 'HOLD SHORT' WITH THE WORD 'POS' THAT HE RARELY USES. IF ANYTHING; HE WOULD MOST LIKELY HAVE SAID 'POS AND HOLD' WHICH WOULD HAVE CONTRADICTED THE INFO FROM THE PHONE CONVERSATION WITH WSSS CTL. I'M AT A LOSS TO EXPLAIN THIS SITUATION. THOUGH THE REMAINDER OF THE FLT WAS UNEVENTFUL; THE ANXIETY OVER THIS SITUATION WAS APPARENT. WSSS CTL SAID THEY WOULD RPT THIS DIRECTLY TO OUR COMPANY.