SUPER MD80 TAXIING TO RWY 9L WAS CLRED TO HOLD IN POS FOR RVR IMPROVEMENT. LATER CLRED TO HOLD PAD ACCOUNT TFC RWY 9L. MADE 270 DEG L TURN TO CLR RWY AND DID NOT GET ACFT BEHIND HOLD LINE. ANOTHER HOLDING ACFT POINTED OUT AND WAS ABLE TO REPOSITION THE ACFT BEHIND HOLD LINE. WAS CLR OF RWY BUT NOT IN THE DESIGNATED AREA. VISIBILITY VERY RESTR BY FOG.

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TAXI TO RWY 9L. FOG REDUCED RVR BELOW TKOF LIMIT AFTER TKOF CLRNC RECEIVED. DISCONTINUED TKOF AND HELD ON RWY 9L PER TWR INSTRUCTIONS. 4-5 MINS LATER; VISIBILITY STILL BELOW LIMITS; INSTRUCTED TO TAXI OFF RWY WITH AN IMMEDIATE R TURN TO HOLD WITH OTHER ACFT AWAITING TKOF. UNSURE OF ABILITY TO DO IMMEDIATE R TURN; DID 270 DEG L AND EXITED INTO RWY 9L/12 PAD BTWN THE INTERSECTING RWYS. TWR ASKED IF WE WERE CLR OF RWY 9L AND FO REPLIED WE WERE CLR (MEANING PHYSICALLY CLR OF THE RWY). ONE ACFT LANDED ON RWY 9L SHORTLY THEREAFTER. NUMEROUS ACFT HOLDING FOR BOTH RWYS 9L AND 12. ONE OF THEM REMINDED US ABOUT THE HOLD LINE AND THAT WE WERE NOT BEHIND IT. WE CLARIFIED WITH THE TWR THAT WE WERE PHYSICALLY CLR OF THE RWYS BUT PAST THE HOLD LINE IN THE 'PAD.' WE WERE UNSURE THAT WE HAD ROOM BEHIND HOLD LINE (POOR VISIBILITY AND ACFT HOLDING FOR TKOF) TAXIED AT ACFT HOLDING FOR RWY 12 AND WAS ABLE TO REPOSITION BEHIND HOLD LINE. NO FURTHER INCIDENTS. WHEN INITIALLY TOLD TO EXIT RWY 9L I WAS UNSURE THAT A R TURN WAS POSSIBLE DUE OUR RWY POS; POOR VISIBILITY; AND ACFT HOLDING FOR TKOF TAXIED INTO PAD; GOT AWAY FROM RWY 9L; BUT WAS CONCERNED WITH CREATING A NOSE-TO-NOSE BOTTLENECK WITH OTHER ACFT DID NOT REALIZE HOLD LINE PROB IMMEDIATELY. IN RETROSPECT; SHOULD HAVE REFUSED TWR'S GENEROUS OFFER TO REMAIN AT THE FRONT OF THE PACK AND REQUESTED TO TAXI DOWN THE RWY TO EXIT. CREW THOROUGHLY DEBRIEFED INCIDENT; EXPLORED ALTERNATIVE COURSES OF ACTION AND REBRIEFED PART 1 DEFINITION OF CLR OF RWY.