B732 PIC TAXIES ACROSS BNA ILS HOLD SHORT LINE; THINKING THAT THE FO MEANT THE RWY 31 HOLD SHORT LINE. ACR ON SHORT FINAL REQUIRED TO CIRCLE TO RWY 2.

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WE BEGAN OUR TAXI AT BNA AND I CALLED GND CTL APCHING SPOT X. WE WERE CLRED TO RWY 31 AND TOLD TO HOLD AT THE ILS HOLD SHORT LINE. I ACKNOWLEDGED THAT WE WERE CLRED TO RWY 31. AS WE APCHED THE ILS HOLD SHORT LINE; I ADVISED THE CAPT WE WERE INSTRUCTED TO HOLD SHORT OF THE ILS LINE. HE SAID ILS APCHS WERE NOT IN USE AND WE COULD USE THE RWY HOLD SHORT LINE. I THEN ADVISED HIM THE GND CTLR SPECIFICALLY TOLD US TO STOP AT THE ILS HOLD SHORT LINE. HE THEN STOPPED THE AIRPLANE ABOUT 40 FT BEYOND THE ILS HOLD SHORT LINE. THE TWR CTLR ADVISED US THAT BECAUSE WE CROSSED THE ILS HOLD SHORT LINE; THEY MADE ACR Y CHANGE FROM RWY 31 TO RWY 2C. AFTER RECEIVING THE TAXI CLRNC; THE CAPT SAID OK. WE DID NOT SPECIFICALLY DISCUSS 'ILS HOLD SHORT' LINE VERSUS 'RWY HOLD SHORT' LINE. WITH THE WX BEING CLR SKIES AND 10+ MI VISIBILITY; IT IS UNUSUAL TO STOP AT THE ILS HOLD SHORT LINE. I THINK THAT IS WHY THE CAPT WAS THINKING 'RWY HOLD SHORT' LINE. IN THE FUTURE; WHEN INSTRUCTED TO USE TO ILS HOLD SHORT LINE; ESPECIALLY DURING GOOD WX; I WILL RPT THE CLRNC TO THE CAPT AND WAIT FOR HIS RESPONSE; SIMILAR TO AN ALT CLRNC. SUPPLEMENTAL INFO FROM ACN 497663: I WAS PAST MY ASSIGNED POINT; BUT SINCE I WAS SO FAR BACK FROM THE HOLD SHORT LINE; I WAS AND AM; MYSTIFIED AS TO WHY THE B737 ON FINAL WASN'T ALLOWED TO PROCEED TO A RWY 31 LNDG. IN MY MIND; I WAS GOING TO GO TO THE 'HOLD SHORT LINE;' BECAUSE THE WX WAS CLR. I ASSUMED THAT THE TAXI WAS CLRED ALL THE WAY TO THE RWY. ADDING TO MY MENTAL 'HEAP' WAS THAT IN MY EXPERIENCE AT BNA I HAD NEVER BEFORE BEEN INSTRUCTED TO HOLD AT THAT PARTICULAR APCH LINE SPOT. AS I APCHED THE APCH LINE; MY FO INFORMED ME THAT I WAS SUPPOSED TO STOP AT IT.